I get being upset in the moment, it was at the press conference it just felt he was coming off as a bit of a doosh. He's basically saying they shouldn't have called it because it was a cool play, c'mon man, comes off as being very entitled. If he should be mad at anybody he should be mad at his teammate. It wasn't like he was even trying to make the case that he wasn't offsides, he was by a lot, it wasn't like he had a toe on the line. The refs have even come come out and said they will help the WRs to line up if they they look over for some guidance, they don't do that for defenders, but the catch is the WR has to look for the help. Don't expect the refs to stop play to make sure your guys are lined up correctly or just let it go when the player can't even be bothered to check.
It just comes across as whiney when your guy screws up and expect the ref to not call it. I hope Mahomes saved some of that anger for his teammate, he's the one that deserves the hate.