Certainly a possibility eh, doesn't seem to be an obvious female equivalent to Oscar, given how odd he is.
I'd say there's a difficulty in writing a deep friendship between guys and gals without the relationship question being raised. There's an intermittent intimacy in super close friendships like that and as such it overlaps with much of what makes up actual relationships.
I do wholeheartedly agree though, people really seem to struggle with the idea that people who are contantly in close proximity to each other really have to form some kind of relationship to get by. Cartoons that show that well are almost always fantastic, that was certainly one of the biggest draws to the original Avatar series and one of the biggest weaknesses of the sequel.
Star vs. is one of the few shows in recent times that's nailed that intimacy between friends. It's also done it in a platonic manner, which is why I'd actually prefer it if Starco doesn't happen, to show that deep friendships can be done and have their own value, unique to the typical relationship race.
Good portrayal of relationships too is far too rare in any media sadly, creators almost seem afraid to show the downs that naturally encompass committed relationships. They also don't tend to even do the courtship phase very well, attraction is usually only shown as a one way thing; one person striving to convince and capture the other side. Chemistry can be sidelined pretty hard in that respect, and characterization forced to fit the relationship framework is a possibility. I'm hoping they don't go that way with Jackie, hopefully she gets her own stand alone personality that Marco has to actually discover and interact with. I want to see him learn what she's like and react to it. Will he still like her as much? Will they have good chemistry? Is he willing to change to accommodate her etc.