I personally bought it because it looked like some innovation on the 2D Mario formula, which -to me- had become stale with New Super Mario Bros. The little I played multiplayer, I didn’t find it particularly great, mostly when players have very different level of skill. I think this might be a case of “correlation does not imply causation.” But what do I know 🤷
What’s a safe bet is -as the article says- the movie definitely helped boost sales, as it did with older titles like Mario Kart 8 Deluxe.
I am glad other people had fun with the multiplayer, but I agree with you. Playing Wonder with my young kids was just painful, especially compared to Kirby, Yoshi, and some others.
I like the single player though.