As I understand it, the whole alternate universe in which fat adult Monica existed was meant to show that the paths they had taken had led to self growth and this without those paths they were all a bit shallow and magnified 2d versions of their characters.
Joey was an arse because he'd made it big too soon. He had to learn humility over the course of the two episodes whereas he learns that gradually and is therefore more grounded over the series.
Phoebe had completely lost touch with who she was. She had lost her caring spiritual side and so all that was left was her bitchiness.
Rachel was shallow, spoiled and had no drive. No career, shed reverted to an airhead.
Ross was whiny and entitled. About the same for the series lol.
Chandler had built a wall of humour around himself and so hadn't developed elsewhere. He had no confidence and was a loser.
Monica doesn't stand out in this. She was naive and fixated on food because she didn't have much else going on in her life. I don't see that she was particularly picked on anymore than the others.
It's meant to illustrate they were all where they were meant to be. Hence chandler and Monica end up together. Ross and Rachel leave their partners. Phoebe loses her job. Joey gets away most lightly I guess.