100%. I feel like an emphasis placed on a main cast is what strengthens the entertainment value of side characters; adding more and more characters we're supposed to know intimately makes every character less important. If you ask me, what made Marceline such a fun side character was largely the juxtaposition of her basically being to Finn who Finn is to everybody else(She wasn't egregiously evil or malicious, she was just too hardcore for him), and when it was shown she also has weak spots and insecurities it made her from a minor antag to an interesting frienemy. As she became more and more prevalent I couldn't help but think she was redundant and unlikable.
It was also super frustrating how Finn never got a happy ending with anything, I think they really thought ending every arc with some form of fuck you to him made it "deeper" and "mature" or some shit, when really it just became frustrating to watch. Wants a girlfriend, gets rejected at every turn. Meets his dad for the first time since he was a toddler, he's a massive prick with no willingness to change. Meets his mom after sailing halfway across the world to find her, she's dead and also a minor antag. Wants to see humans because he's never in his entire life before(Other than Susan), they're incredibly bizarre and he can't relate to them at all, making living with them impossible and he has to go back to his other life.
I would've wanted to see him happy, and grow in ways other than disappointment. I mean, he has an insatiable curiosity, unwavering heroicism, pretty awesome power, and a need for excitement an adventure. Could they not find anything to do with that?