I don’t think it deserves the hate it gets. It’s a genre targeted at a younger audience, and people don’t get that when you get older, the stuff aimed at younger people don’t appeal to you anymore.
Something you also see a lot in other hobbies in the entertainment industry, is the hatred of entry level stuff. Since battle anime is usually most people’s introduction to anime, people tend to think entry level means not good.
On the other hand, the battle anime fandom isn’t exactly helping their case either. A lot of them are what I call “reverse” snobs where they have the mindset of “if I’ve never heard of it, it’s not good”, and most popular anime are battle anime. So you see where this is going.
The stale nature of the community is also not helping. If you took a 10-15 year hiatus and came back, it’s the same conversations about the same shows years ago, and not much else has changed.