This raises an interesting question on subjectivity.
Just what /is/ good level design to you, for instance?
The Galaxy games are so highly praised because they're unrivaled in ingenuity, creativity, and use of 3D space and mechanics. Their stage designs pull out every conceivable trick in the book, and usually each level is its own unique idea that's almost never repeated through the entire game.
Their camera system is also pretty objectively just about the best camera so far in a 3D platformer in terms of clarity and how rarely it gets confused or loses a good perspective, and most of the time, manages to do it automatically without the player constantly adjusting it.
In terms of character physics, Ratchet & Clank is also below not just Mario Galaxy, but a number of other 3D platformers (including some N64 games). Ratchet has just never had any weight and tends to feel as if he is sliding along surfaces rather than attaching to them. It's not /bad/ but it's not great either. Kind of average.
Now, off the top of my head, one thing I can think Ratchet does have in its favor, is spectacle. Ratchet's sci-fi setting is used well with stage layouts organically worked into the art direction, especially in the PS3 games. I don't want to make assumptions, but is it a thematic thing for you? Is Ratchet's world just more appealing than the cartoon that Mario is?
The problem with calling Mario Galaxy "just mediocre" and saying "if it wasn't Mario, it'd be bashed to hell" is that it's a tall, ridiculous sounding statement because Mario Galaxy is a series that attracts people who /don't like Nintendo games/. It's not about the character, or Peach and Bowser, it's about the creativity, control, level design, music, and experience. That's why the Galaxy games have been so universally praised including by those who are on a mission to despise the Wii and hate Nintendo.
If Mario Galaxy was The Adventures of Cardboard Box and everything was featureless cubes, it'd still be an amazingly designed platforming experience.
So yeah, there's a longer and more reasonable response I think, to why people are going to laugh at someone who comes in and starts saying "bah, Mario is stupid and overrated and is actually a terrible platformer! Almost every other platformer that's not Mario is far superior!"
And this is not directed at you personally, but it's also pretty common for anyone who says a Nintendo game is superior to be called a "mere Nintendo fanboy" just because it's Nintendo, regardless of the game's merits. Nintendo isn't Cool for School like the bros enjoy, and is supposed to only be for Pokemon kids and "neckbeards". For some guys, they'll never accept that anything with Nintendo on the box will be for anyone other than "fanboys". Just as some people think all Blizzard games are for "fanboys", all Valve games, etc, for whatever reason the individual has an irrational problem with a brand identity.