Good gameplay is important to critical reception. There have been tons of games that had great graphics, cool characters, good music, etc... but got poor reviews due to subpar gameplay. I could see this game averaging in the 70s if the gameplay isn't up to snuff. Would that be considered a failure? Probably, I imagine the devs would be aiming for higher than a Metacritic 70.
Maybe not a guarantee, but a Mario game has a much higher chance of selling well than a lot of other games. Almost every Mario game on Switch has sold over a million copies. Even super niche stuff like Mario Kart Home Circuit, or games that fans online have very vocally complained about like Mario Strikers: Battle League. I know Ubisoft was disappointed in the sales for Sparks of Hope, but I don't think we know the actual numbers. And in that case, a big part of its slow sales can be attributed to them shooting themselves in the foot by constantly devaluing their games and teaching people to wait until they are at least 75% off.
I guess I'm just confused by why you think Splatoon, Arms, and Astral Chain "live or die by their gameplay" but not Princess Peach Showtime? It sounds like you are saying that doesn't apply to Peach because people are inherently interested in a game starring her just because she's an existing character in a popular franchise? Compared to a new IP that has to work harder to impress people to catch their attention? Yeah, I suppose that's true.
Personally, I'll only buy a game if its gameplay looks good. The one exception is I'll still buy certain visual novel/adventure games that are super light on gameplay if they have a really compelling story (Ace Attorney, Steins;Gate, AI: The Somnium Files). But I would never buy a game like that with a story that wasn't appealing cause the "gameplay" of picking dialogue options and such isn't inherently fun on its own.
I already know the Princess Peach game isn't gonna have a compelling story, hence why, for me, it lives or dies based on its gameplay. A lot of people will be interested in and/or buy this game just cause it has Peach. I'm not one of them. And based on the comment from JR436 earlier (quoted below), there are others out there like me who are saying the same thing. So clearly Nintendo is missing the mark for a decent chunk of the potential target audience with the way they've been marketing this game.
Side note, I had a good time respectfully debating you, but I'm off to bed now. Have a good one!