I think that the argument that Ridley was too big is valid. Why? Because to include him in Ultimate, he was shrunk down. Ridley at his canonical size is significantly larger than the other fighters, which makes him ineligible as a fighter.
That said, I'd prefer to have him in than not at all. I've always wanted him in, and my solution was just to make him smaller. Obviously, this is something that Sakurai was reluctant to do, as somewhere during or after Brawl he really goes all out in making the characters as accurate as he can possibly can.
(Naturally, some people will counter with him being a smaller size in the original Metroid, but that was retconned with Zero Mission.)
I also feel that Ridley shrunk down doesn't open the door for future large characters like Petey Piranha. I think Sakurai made an exception for Ridley due to fan demand, but I feel he would be reluctant to do it to any boss character that's very large.