I think there is a little bit of bandwagon thing happening with this. It's an unfortunate combination of 1. a lot of people not liking his performance/movie 2. the studio focusing the marketing on him without him or us knowing how little of him will be in the film 3. the juvenile method stuff
Now 3 is nothing new with Leto, he was always doing weird stuff before this movie but it never overshadowed his performance. He was so good in DBC no one made fun of him even though he made it very hard not to by mentioning the fact he waxed his pubes off on every occasion ( :/ ). But now with all of this going on and how little of him was in the film and how much he was promoting it it is ridiculous he did all of that for so little. So people don't go 'well Leto is a good actor and it's only that Squad thing that was bad' when they hear he is cast, they go 'oh not him he will ruin the movie! he sucks! he is a terrible actor! *method acting jokes*'
I'm not so sure he can come back from this as an actor and it would take a lot to turn the majority's opinion of his Joker. I read a lot of websites, boards, gossip sites etc. and he is one of the few people who seem to be universally ridiculed everywhere