I'm definitely biased to some extent since I've always preferred Star, but: personally I always saw Star and Marco as complimentary, with the strengths of one balancing out the weaknesses of the other, with neither being the "mentor" to the other. while that does mean both being featured together in episodes can and did serve the story well, it also means neither one needs the other to carry the story since both characters are developed enough to go solo (Star taking precedence since she is the main protagonist)
but I do think they fucked up hard with Marco this season. making him a squire on Mewni seemed like an obvious way for him to have a new goal and purpose in life, rather than "just" wanting Jackie or being in a go-nowhere karate class he's dissatisfied with. it felt like a very natural evolution of his character development when just the episode summaries about him becoming a squire were available, before they actually aired and I realized they took it in the worst direction possible.
I really really hate that he's Star squire. their relationship is absolutely ruined and, honestly, one benefit of more solo Star episodes is that they don't have to highlight that. he has no goal, he was somewhat forced into it, he's not getting anything out of it that he wouldn't have gotten already if they'd just stayed friends (of course Marco would help Star anyway!), and on and on. what the fuck is Marco even doing during episodes like Bogbeast or Starfari? I'm not sure even the writers knew-- at least if he continued being Lavabo's squire that wouldn't even be a question
I'm not sure Marco was ever AS important to the plot as Star, but they could definitely be doing more with him this season than they are. I'm not even a huge fan of the character and this plotline/use of him feels somewhat insulting to him and his development, mainly because I'm not sure if the crew realize just how fucked and how much of a downgrade it really is