Yeah, they fucked up big time. They had quite an amazing change of phase during season 2. They were apparently breaking the mold that Daron promised, about making the show different to the usual stuff.
The male friend the protagonist had was developing away from her, Marco was getting puss and growing as a character and it wasn't Star's doing, I don't know why or who decided that was bad, the numbers don't lie either. S1 had a low kick, then with season 2 there were specific episodes that brought the show to a constantly increasing audience:
Bon bon the birthday clown (literally took the show out of the low ratings it had)
Running with Scissors (it continued taking the show higher)
Just friends (First high audience peak season 4 finale didn't catch with it)
Starcrushed (got the real 100% audience)
Battle for Mewni (also got the 100% audience)
I don't know who decided that was bad, or why, but I can only asume Daron was spiteful that her self-insert main character was not getting the attention she expected. The worst part was that Toffee the one good main antagonist they had was killed in the worst way possible, as if he was a joke.
But it doesn't end there, Eclipsa, the Dark Queen of Magic, the one considered the worst enemy possible, the one whose chapter Glossaryck (magic itself) considered of Cosmic Danger, was turned into a "cinnabon roll who loved in a land of racists" they shouldn't have change her status as enemy, now in retrospective we can be sure that the series should have ended with Battle for Mewni and Daron could have had her "open ending" that she so much wanted.