I often hear that a writer should not write something that is not tightly linked to the plot. "If you can narrate it without it, drop it from your story" - that's what I see.

However, is it really bad to include something just for fun or just because it conveys the rare (exotic and interesing) idea? Should I not include such a thing in the novel?

I'm guessing without these things the novel might just become a non-artistic book, like the scientific (not a popular science) one, but just the one that describes something unreal. Unscientific science.

Thus I guess the answer is not positive. But how much should a writer deflect from the plot? Is it OK to devote a whole chapter (a few pages) or a couple to it? Is it OK to have an unnecessary frame story (within given setting) just because the story itself might be cool (within the context of a novel)?

However, is it really bad to include something just for fun or just
because it conveys the rare (exotic and interesing) idea?

It's not bad. Truth to be told, many successful authors do it to an extent.

What you are describing is akin to the process of worldbuilding:

As Matthew Dave said, sci-fi is a major example of it. A lot of short stories (I'm reminded of Asimov and Ted Chiang) are built around "exploring an idea" rather than exploring a plot, or a character arc.

But the same could be said for other genres of novels. Mainly it's something you'll find wherever the author is building a fictional world, so fantasy and it's many branches are all culprits, but you'll find worldbuilding efforts across a wide variety of genres (I'm willing to argue that horror, distopian, alt-history and historical novels all fall into the list).

Exploring exotic and interesting ideas is usually fun for the writer. And it can be fun for the reader too, if done well, because it engages the reader in an intellectual level ( No suprise there's a whole SE for that ).

Let's say you introduce FTL travel in your sci-fi novel. Maybe it's not a core element of your plot: it serves only to carry your characters from point A to point B. You may just tell the reader "yea, they got FTL" and move on. But most novels don't cut it so short.

Seeing how a writer takes an interesting idea and expands it into a working enviroment is engaging.

But, balance is key.

While it's true that the readers may enjoy your ideas, some will want to see the plot go forward. Delving too much on exploring facts and ideas risks to bore or alienate part of the audience. So, think about what kind of readers you want to keep in: the action-thirsty ones or the more speculative ones.

And (again as Matthew Dave already said) learn when and how to interleave plot and setting without making your novel worse.

I'm reminded of China Miéville's "Perdido Street Station": it's a really great book with a really original setting, but the author has the habit of starting almost each chapter with a long description of how the streets of the main city look like, where are the squares, how building looks. He's very good at doing it, but personally in some chapter towards the end I just wanted to see the plot unfold. On the other hand, the friend who suggested the book to me enjoyed those descriptions wholly, so to each their own.

One last thing:

I'm guessing without these things the novel might just become a non-artistic book, like the scientific (not a popular science) one, but just the one that describes something unreal. Unscientific science.

You may want to explore differents formats. I see those "unscientific science" descriptions that you talk about more suited to short stories (as I said, it's not unheard of in the sci-fi genre).

In a short story - almost like a scientific article - you could dissect an idea without boring the readers, in a format like "What if x - then y".
Longer formats, like novels, will probably require a plot able to stand on its own.

Some authors do include pieces of "scientific like" description of non-existant things. In the Thirteen lives and a half of capitan Bluebear, Walter Moers inserts encyclopedia pages describing creatures of the world. Are they relevant to the plot? Eh, not really. In Ensel and Krete, it gets even worse! I've also seen some italian authors do this (in parody and satirical genres, like Stefano Benni's works).

Is writing solely about writing a plot?

No. A plot is needed, but writing is about far more than the plot.

It is (IMO) impossible to write a good story without conflict going on. If there is no problem facing the protagonist, no momentous decisions, nothing they want but cannot reach -- I don't see a story, I see a slice of life or a description of something.

But No, writing a story is not solely about the plot. It is about entertainment of the reader. Your job is to assist the imagination of the reader, so they can be entertained. Things that have nothing to do with the plot are described all the time, things that could be excised without affecting the story. The appearance of most characters is not important to what happens, most descriptions of the setting or culture are not important to the story. JK Rowling doesn't have to describe the hallways or moving staircases or the spells gone wrong in class, they don't truly bear on the plot.

But all of that stuff is entertaining, and that is the only reason anybody buys your story or wants to read your story, so they can have some fun in their imagination (assisted by you).

The purpose of plot and conflict in a story is to create outstanding unanswered questions in the reader's mind about what will happen in the next few pages, in the next chapter, and by the end of the book. Conflict is interesting, whether it is the momentary conflict of disagreement in a conversation, or the epic conflict of risking everything to save the world. A plot, an overarching intent to make something happen (or make it not happen), is a framework that you can decorate with lots of conflict and writing about an interesting place, or interesting people, or interesting talents, all with the intent of entertaining the reader. The plot is their excuse to travel, question, strive, etc.

So while you will likely have a plot in your story (readers expect something), by no means should every line be tied to the plot, that can be a boring story. But every line should be part of something entertaining, not just to you but to the reader.

Science fiction (hard sci-fi) often delves into asides about fictional science and technology just to have fun with the concept. While you should ideally keep up a pace that makes sure the plot is always going, a good book that isn't set in our world should adequately explore the setting so the reader at the very least understands the context, and at best takes an interest in the world for its own sake.

A good writer knows how to weave plot and setting nuggets together without impeding the pace of the novel. Knowing the balance is part of the skill of a writer.

Moby Dick is an undisputed masterpiece. It includes chapters on the technical aspects of whaling and the reader learns how much oil can be harvested from a dolphin and any other of the species out there.

I love that book, but Melville chose to include chapters that expanded on the subject that his characters knew so well. A reader in Minnesota would learn about something they never might otherwise, but did it serve the plot?