I think YABU purely because why would you sneer about someone else's harmless interests? The football comparison is a good one. We all like different holidays and hobbies.
I'm not a "Disney adult" and actually the only one I do know irl (through work) pisses me off no end for unrelated reasons. But I do respect that they like what they like and embrace it fully without worrying about the judgment of others (like this thread).
We've done disney 3x (over 10 years) as a family, and would do it again. It's one of a range of holidays we've enjoyed. I wouldn't go without kids I don't think, but i have bloody loved it every time - seeing the magic in my children's faces, and the way you can utterly let go of the outside world and its stresses and relax as a parent, I've genuinely not found that elsewhere for such a prolonged period, including on other great holidays (possibly the only one which came closest was a kinderhotel because the kids' needs were totally catered for, but I wasn't insulated from the sometimes-miserable outside world the way you are at disney).
I love the fireworks, the parades, the rides and the restaurants, the beautiful attention to detail in the theming of the park. It's clean and happy, and it's incredibly easy for me as a parent. I love seeing my children interacting with the characters. Loads of that is tied up for me in vicariously loving my children's enjoyment (the childfree holidays i fantasise about are usually more hiking holidays or city breaks), but I can definitely appreciate why adults without children could enjoy all of this for themselves.