When I initially watched the first couple of seasons of Star VS the Forces of Evil, I had a very positive reception of the series. But, then just after the Battle for Mewni... The episodes took a SERIOUS nosedive in quality. It's a real shame, considering how good the series USED to be. Now it's become mediocre to the point that I can hardly even tolerate watching it any more. Well, the series has 3 main problems now that need to be discussed to answer the question of why.

1. The Animation. It's not bad, but it doesn't fit the show anymore. The animation just had this looseness to it before that fit what was at the time a good, rather off-the-wall episodic show that built itself around following the characters around their lives rather perfectly. Now it's lost that loseness, and instead has become way less fitting to how I perceive the show should be. It's become a show with more computer-generated animation, and less of the earlier hand-drawn animation, which for a show specifically one like Star VS the Forces of Evil, doesn't really work...
2. The Characters. I know that's not specific, but the characters in general are losing charm, and a lot of the enjoyable secondary characters are losing relevance as the show goes on. Star used to be a fun-loving, carefree teenage girl from another universe where literally everyone grows corn for some reason. Now she's become stern, and definitely not fun-loving. I honestly can't even really describe it much more than that, but it is a TOTAL turnaround from her original character, and it's BAD. As for Marco, I don't understand what's happening to him either. It's similar to Star except of course the opposite. He used to be a good counterbalance to Star's whimsical spontaneity. Now he's just becoming... A blank slate, who's actually lost a lot of his maturity... It really doesn't make sense at all in his case, because he aged 15 f***ing years in an alternate dimension... HE'S SUPPOSED TO BE MORE MATURE, NOT LESS!!! As for secondary characters, I don't even remember seeing most of the ones that were actually well-conceived. Instead we just get Pony Head. ALL THE TIME. And also some of Tom, but let's face it considering he's a demon, he's really, really bland... There's nothing about him that makes him interesting... We also have background characters constantly repeating bad, annoying one-liners more often... I swear I'll slap that... thing that keeps saying "CAMERAPHONE!" and taking selfies. IT WAS ANNOYING THE FIRST TIME AND NOW IT'S JUST DOWNRIGHT INFURIATING!
3. The Plot. This is the most egregious part of all. Star VS was never centered around it's plotline before. The show mostly made it an afterthought until around mid-season 2. AND IT WORKED REALLY WELL! The series originally worked this way because the characters bounced off of each other so well that they could literally use that to make episodes that were absolutely amazing in their own right. They made a plot-based show work well for half a season, because the plot was an interesting one about Toffee's network of minions and stopping Toffee from getting control of Mewni. But now what is it about? SHIPPING? Trying to force Starco? F*** NO. IT WORKED FINE WHEN IT WAS A SUBPLOT. But now that it's the MAIN PLOT POINT AS OF EARLY MID SEASON 3 instead of... you know... ECLIPSA... But even then, that shouldn't even be emphasize on as much as the SHIP. Seriously, this ship as of far has been the main focus of more episodes this season so far than the ENTIRETY of the last season... And I only got a THIRD of the way through before I had to stop it... 

It's really a shame that a 9/10 show became a 6/10 show so quickly, but it's no wonder that it fell so far. With the poor characterization, the plot becoming ABOUT a forced ship, and the animation not fitting the show as well, it's pretty different from how it used to be. Where did we go so wrong...?