That kind of depends on what you mean by 'the plot'. Could the story largely continue in the same direction without her? Sure, but you could say the same thing about Lillith. Her inclusion adds spice to the dynamic between Becca, Abby, and Frank, but without her the story could still be pretty much exactly where it is now. (I'm saying this as someone who adores Lillith's content, and wouldn't want it removed for any reason)
Mei has had significant interactions with Frank, and there's been a fair amount of game time devoted to explaining her relationship with her parents, and how she's always had a crush on him and such - I was certainly surprised to learn she wasn't considered a "main girl" based on this.
Finding her boring is a completely valid personal opinion, and I'm sorry you don't enjoy her, but I certainly get why so many people are on the "Mei is a Main Girl" bus. Honestly, the only one so far who really feels like a complete side girl is the Model/Waitress, because she doesn't have much to add beyond sex and sexiness yet. Our favourite Lesbian Couple, on the other hand, give Frank someone to talk his problems through with, and add complications to their general lives in the "Rich Parents" and "Need a baby" storyline.
