I enjoyed listening to Messiaen's music long before I understood a thing about it. His Quartet for the End of Time grabbed me from the first bar and never let go. I understand it extremely well now, esthetically, technically, and theoretically, but that was never, ever essential, let's be clear.
A few other things to understand: no, this music was not written to be deliberately ugly. In fact, a notion of ugly is actually an acquired thing. I myself do not and did not find Messiaen's music ugly, ever. What it actually was, was a desire to enable creativity ito find new modes of beauty: that very much so, as opposed to treading over the same extremely well-worn paths that everyone already knows about.
Just about everyone thinks Mt. Rainier is beautiful, and that fact makes a picture of it a potentially easy (in fact, very easy) way to make something beautiful. But it's also totally banal: there's little to nothing new to say about beauty with yet another picture of Mt. Rainier. So, what about a picture of a homeless person? Can beauty be found there? How about a picture of a derelict building? A crumbling street? A vandalized concrete wall? A pile of litter? Are these things only ever irredeemably ugly?
Those are just examples, and even with them the clichés run amok nowadays, certainly. But I hope my point is clear: at some point, the truly creative photographer isn't going to be artistically satisfied with just another picture of Mt. Rainier at sunset with a lenticular cloud over it. It's a copy of a copy of a copy of a copy of a copy of a copy of something someone else did. Not invalid, not not beautiful, just not very interesting or original as art.
So it is with composers, and actually has always been. People were shocked at Beethoven's Fifth Symphony when it was new. Many people found it contemptible and ugly, in fact! And this was in a time when performing any music older than a couple decades was just not even a thing at all. Yet still for many Beethoven was too radical. Many composers pushed boundaries and have been pilloried for it, in fact, from antiquity onwards.
I can name examples of great art that was confusing, baffling, or deemed ugly or awful when it was new, that is now treasured and widely accepted as part of our collective artistic heritage, from any time period you care to name.
So, sure, some people start, or waltz into, a thread like this, and bash the tastes of people, like me, who genuinely, truly, adore the music of a composer such as Olivier Messiaen. And that's just fine, as I'd probably find the musical tastes of someone who would do that entirely worth bashing as well.
De gustibus non est disputandum.
P. S. Kindly stop using "atonal" as a synonym for "music I dislike." The word has almost no worthy meaning; even Schönberg repudiated it as a term. "Post tonal" is far better, because the fact is, we live in a post-tonal musical world. Dissonance has been emancipated (jazz for one would be impossible without it), and the actual, real principles of common-practice tonality as any composer would have understood it before the 20th century (but after the early 17th century) are basically not substantively adhered to by anyone, not pop music song writers, not neo-tonal composers, not film composers, not anyone. Possibly music theory teachers but that's about it. Unless you're perfectly fine with a mushy definition to the word "tonal" that would actually apply to literally all music ever, if you just thought about it for half a second, or would just mean something as vapid as basically just "only the music I like": we can do better.