Sure, but there's a little more to it than that. Actors are instructed what to do. If I see a performance in a movie that I don't like, if the actual performance is good but the writing/direction of the performance isn't something I like, I don't blame it on the actor. McCarthy has been in some bad movies; most actors have. But she's also demonstrated talent more than once and has been in a few critically acclaimed movies.
For me it's not really about who is in it, but rather how they're told to act, what they're given to say. McCarthy isn't inherently awful.
Sure but I'm just saying it's not completely unfounded. McCarthy is good in a lot too so you could swing the other way as well. Wiig I've found consistently funny in every comedy movie I've seen her in. So all I'm saying is in a movie where it's mostly about the characters playing off each other, even if McCarthy is just missing the mark Wiig could easily just come in and pick it up and save it.