I don't think people are saying that. But I think after the success of GOTG which I literally had ZERO expectations going in on, the fact that the trailers make the Joker look like the big bad. And the fact we are getting our first on screen portrayal of Harley Quinn. people have every right to be disappointed.
I mean look at the cast for fukk sakes. Will Smith, Margot Robbie, Viola Davis, Jared Leto. These aren't B team motherfukkers. This ain't Chris Pratt and Batista. these are actors with gravitas.
you are talking about a couple different things i'll address:
1) expectations - people dont know the ss characters just like they didnt know gotg characters, so expectations for the actual characters shouldve been the same....
2) cast - yes, they are well known, and?...the actors in ss killed their roles and met/exceeded expectations in that regard, which most are saying, so how is that a disappointment?....
3) villain - not at one point was the joker sold as the main villain of this movie...that is shyt people ran with in their heads cause the most interaction the majority of people have had with the SS is 'attack on arkham'....they saw him and thought this was going to be a live action of that movie...so instead of judging the movie off what it is, they judging it off what they wanted....and since it's not a live action version of that movie, they are 'disappointed'...