Well; this game is a lot kinder to Marisa than 17.5 was; where she was literally there because she's Marisa so has to be in the game but her route is 100% filler content and 100% non-canon.
The first narration line of the tutorial makes Marisa canon in Touhou 18. It says "The Market God was defeated by the Magician". This is becoming a bit of a trend in .5 games; after 17.5 made Reimu-Eagle canon in WBaWC [Yuuma specifically says Reimu was the one her spirits helped to defeat Keiki] and EoSD [Flan remembers fighting Reimu; but if you choose a route where she fights Marisa she dosen't have any special reaction to her or vice versa]
This game also marks the first time in official canon where Marisa has defeated Reimu... unless we assume all Stage 4's except Nitori don't actually happen. I mean I guess it's possible Marisa didn't fight Reimu but I think we can give her the W here.
re: Marisa canon
The impression I've long had is that with a few exceptions the basic encounters and endings in most games "happened" in some way, shape or form, though not necessarily the way we see them in the games. Most notably, every route in HSiFS has been referenced in a future work, so in that case at least they all definitely happened in some fashion. Additionally, Grimoire of Marisa implies Marisa saw many of the cards featured during specific incidents.
Still, Marisa is more inclined than most to be screwed over, because I think her route in SWR is also non-canon (Her encounter in Yukari's scenario implies she has yet to go to heaven despite the story taking place after where Marisa's would have taken place). She's been long overdue for a big game to herself, and I'm happy she finally got it.