I think the basic impulse is correct, actually. There's a very good chance that a person with this kind of attitude is a poor reader—of literature, at least, if not in general. If you drop them right into the heavy stuff, they'll probably bounce off really fast.
It's a skill you have to develop, like when you encounter a new musical genre. Lots of people get stuck at the "this sucks and is probably bad and overrated anyway" stage and don't open their minds and challenge themselves to understand why people like it, when encountering literature early on, similar to how many people get stuck in that same place for any musical genres they didn't learn to appreciate by age 25.
At best, they end up reading very light fiction for the rest of their lives—perhaps even YA, which is widely read among adults, in fact. At worst they decide the whole thing's a pretentious sham and they're smarter for not having engaged, which seems to be closer to the case here.
Not sure I'd go with YA if you're trying to convince someone of value in literature beyond entertainment, though. But lighter high-school tier literature, maybe. Salinger (but probably not Catcher in the Rye) or Vonnegut are decent choices, being at about the lightest end that'll still (if the reader's open to it) work those muscles. Gotta start small to develop the skills & taste to appreciate the sublime when it's expressed in written words, and to find meaning, enjoyment, and new mental tools that novice readers largely miss—which is why they often think well-regarded literature is boring and stuffy.
I certainly would, but probably not Harry Potter (personal preference - I think "magic" has been done better, and I think "boarding school" has been done better).
So maybe "Earthsea" by Ursula LeGuin, for example.
To an adult who have never read fiction? Yes. You have to start somewhere.
People are just suggesting their own favorite books, but nobody started reading fiction with Anna Karenina. But many started with HP and went on to more challenging stuff.
reading HP is fast and loose fun. the characters are okay, the world is okay-ish, the story is the standard good vs evil, there are some interesting dilemmas and the whole setting makes it a lot more palatable than the classic classics :)
(of course there are huuuuuuge plotholes and anyone with a more than two bits of imagination/curiosity starts to pick at the surface of the world it falls apart, like almost anything with magic in it)
