Bubbline being canon is amazing, I squee'd so much when I saw their kiss in the finale (despite having spoiled myself already). I love how it was implied that they'd been dating for a while now, with Finn saying to Marceline's dad that she's (probably) dating someone and how comfortable they were kissing.
Also, here's what Adam Muto, one of the creators of the show, had to say about their relationship in an interview:
Marceline and Princess Bubblegum’s relationship was one of the most meaningful parts of the show, in terms of representation. What does the positive reaction to their pairing mean to you, and how did it influence their beautiful ending?
It felt like that was another question. It is set up enough by now that it needs a conclusion to feel satisfying. We can’t say what their life will be like from then on forever, but we can show where their relationship is now as opposed to where it was a couple years ago. Marceline is the adolescent forever, and Princess Bubblegum is the workaholic who doesn’t have anything outside of that.
Being on Cartoon Network, alongside shows like Steven Universe, I do think that we felt like we could do more than we could have back then. Rebecca [Sugar, who created Steven Universe,] really did so much of the heavy lifting that I feel weird even talking about that too much.
It was something that is important. When it showed up in the storyboards, it made sense. You just have to do it enough so that it’s not a new story, and then it will just be something that happens. Hopefully it’s not important in that way. Hopefully it’s not something that we even need to comment on, even.
I took that last part as in "We Ship-Teased them so much that this shouldn't really be much of a surprise", which I liked. The past few seasons had Marceline and Bubblegum get really close, so them dating didn't feel like it came out of nowhere.