He is?? I've followed along all of the twitter compilations from every practice thus far (albeit it WAS just reading what others observed) and it would appear that most indications are that, while Josh makes mistakes here and there, he also attempts and makes throws the other 2 QB's either can't or simply won't make. Based on what I've read from Bills Training Camp is that Josh is ahead of schedule (compared to the post-draft projections on him needing a lot of time to develop).
I would think the reason people are "making fun of other QB's" has to do with the praise heaped on Mahomes without ever having produced anything in the NFL, while many decried Allen a bust...also without ever having produced anything in the NFL. My guess is that people are having fun with the fact that Mahomes has had some struggles even though he was anointed by many to be a star in the making.