Well, most of this hate on Celestia is because of memes and rumors and such. Honestly, I don't hate her, I think it's just ridiculous how every single time something bad happens in Equestria, Celestia just kinda disappears with some excuse as to why she can't be there. And the one time she did stand up for her subjects physically, she was pwned by Chrysalis. Also, the whole 'what was happening in Equestria before Celestia's power' is kinda skipped over by the show, so there's not much to go by. If we actually had proof beyond a stained-glass window and an explanation by Celestia about how Discord's rule was so bad, I might believe her a bit more. I mean, seriously, did he physically hurt his subjects or was he just irresponsible. The first reason justifies encasing him in stone, but with his personality, the secomd one is more believable. But I'm getting off topic. The reason I think a lot of the fandom likes Luna a lot is because, even though she has less screentime, we understand her more than Celestia, and have sympathy for her social awkwardness. Celestia just seems... too perfect, too good to really be good, you know? There is too much room for failure for us to trust her. That is why I think that a lot of the fandom likes Luna more. Plus, Luna has an awesome character design.