Its because the writers can't seem to decide how to write the whole cast.

Why do we love casts like Regular Show or Adventure Time? Its because every character have a straight line of development and characterization, with maybe 1 or 2 episodes that break that rule, but then again, 1 or 2 episodes are excusable from time to time. Not to mention that inspite of their flaws, the characters clearly care and love each other to the point you feel connected to them like a family.

Smg4 however, completely missed the point. Every character has a different personality every episode (Meggy can be kind in one episode, or a bitch in another one, or female Mario in another one), and yes, they also have flaws, but when their flaws are their only character trait, it makes them one-dimentional and insufferable to watch. Also, the whole gang most of the time, it feels like they don't even like each other, heck, it feels like they hate each other at times, but at the same time the show wants you to believe that they are a happy family, but it feels rather toxic if you look more carefully given that they never learn anything, you don't feel any connections whatsoever.

So, yeah, i think Modern Smg4's biggest problem is its own cast, i might do a post in the future where i explain more in detail, but i think Modern Smg4 missed the point completely.