It's a really big thing. AT could only use the "Finn does stuff for, or acts weird around PB because he likes her" so many times, and once that decided to take romance off the table their characters just had no real chemistry, you realize that Finn's motivation for being best friends with PB over being casual friends like he is with other princesses is entirely based on his crush, and their attempts to continue their friendship and PB's major role in the show after that required major shifts in her characterization and relationship with Finn, and the result were really inhuman and impersonal interactions between those two.
There are a lot of other problems with nuAT, and I'm not going to go with the meme of blaming it entirely on Marceline and PB, but having main characters who have no real screen chemistry is fucking cringey and even sort of exhausting to watch, especially when you combine it with just how poor overall the dialogue in the newer seasons are. They're just very dishonest takes on the characters as a whole and a lot of that can be blamed on the fact that many things in early AT (which I love) were not built to last, and were far from versatile. They shouldn't have pushed the show to something it never had the depth to sustain