Sonic Colours is very well received, and at 30fps

Many mice and keyboards on PC operate at a significantly higher rate than the games rendered output framerate... The result is... 30fps is still 30fps, which is okay for some games... But not okay for others, thus this isn't a "new" thing by any stretch of the imagination.

Still. I would reserve judgment untill giving it a go in person, if it's a stable framerate with low input lag and good frame times, then it might feel okay.

or the undocked mode could have lower poly count, less draw distance etc. hence not graphically the same.

But I havent checked if there is any outlet which provides info on this so maybe i am mistaken.

You can see from the screenshots that Sonic's effects and geometry are more advanced than in Mario. It's taxing on weaker hardware while not looking significantly better as a whole.

Mario was built with weak hardware in mind, so techniques to obfuscate low quality assets are built in from the start. Sonic wasn't built for low end hardware so they would have to put a significant amount of effort into the game to rewrite how the physics, effects and visuals work. But why put work into a game on a console where 720p and 30fps are considered just fine.

1st party games on the Switch will always run better for roughly the same visuals than ported 3rd party games. Developers might be more inclined to optimize their games for a specific platform if people had higher standards, but they don't.