Stagnant in the definition of the word. Mario platformers don't make any changes or push any interesting concepts forward anymore.
I'm playing Superstar Saga right now, and the Mario there is bursting with visual personality, on top of being in an interesting, hilariously written, silly game. Mario platformers since 3D Land have basically just been NES games with bigger budgets, and this homogenizing has poisoned most of the sub-series of the franchise. Paper Mario is dead, Yoshi games are eternally spinning in place as retreads of Yoshi's Island, there hasn't been a good 3D Mario since Galaxy 2, and that game was about taking out the interesting story bits of Galaxy to make things a lot more samey and toothless.
You're saying this mostly after 1 or 2 games in a series in this current generation. You're likely not getting 3D Land/World next gen, you're getting something else. 3D Land/World are good 3D Mario games though, just not your cup of tea. Paper Mario had missteps and I seriously doubt it's dead, for all we know it's going back to its roots by next game or somewhere else, who knows. Yoshi just finally got a good game from a good [feel] company by sticking to its roots and having good level design.
Of course Galaxy 2 is going to feel "samey," it's a direct sequel to Galaxy 1. It's using ideas that weren't able to make the first game and expanding on existing ideas to a stronger effect. It's more varied, levels feel more complete, and it's my favorite of the two.
I also hear that Mario & Luigi: Paper Jam is really humorous and well written too. I've seen plenty of screenshots make their rounds on Twitter, especially of Princess Peach and Bowser. Got genuine laughs out of me.
Mario reinvents himself in his core titles every generation, you have no idea what concept they'll even use for the next game. You said the last good 3D Mario was Galaxy 2? Well before the Galaxy games there was Sunshine, and there are plenty of people who do not like that game and say it's one of the worst 3D Marios, yet Galaxy surprised everyone a generation later.
The Mario series that needs to change up more like the rest is the New Super Mario Bros. series. Like the visual style. It's too bland, and could do well with something more stylized, even cel-shading would probably make it look much better.
I find Mario, as a character, incredibly hollow. He has zero personality anymore - and yes, he
once did have a personality (back in the 80's). Then you get the argument that the paper cut out cartoonish character can't have personality - well what about Pixar? Pixar make great characters that even adults can relate to...
Here's Mario's story about how he ended up in the Mushroom Kingdom. This has been completely forgotten by Nintendo.
https://www.youtube.com/watch?v=9osevICCNiI
It wasn't forgotten, it was retconned in Yoshi's Island in 1995.
