Maybe I’m wrong about it being the controls, but when Mario lands he does slip even without holding forward. I’m not doubting that’s not your experience, but it is honestly mine, I don’t know what to tell you. He seems to slip and slide all over the place. I think it’s that he almost feels too “light” in this game, every movement even when intentioned to be slight just seems to send him flying all around the place. In other Mario games he feels heavier, and I don’t really have to do anything other than think what I want Mario to do and he does it; if I want him to do a little jump or jump forwards little, he does, and vice versa for higher or longer jumps. It’s almost as though I merge with the controller, however with sunshine this seems lost. Again, I’m not putting the game down and I’m happy enough to admit I’m just probably not “getting” it, but as I stated I’ve never had to work hard or even at all to get to grips with movement in other Mario games, and he really does slip or slide when he lands without me touching the stick. I’ll press on a little longer and see if it improves, but so far, not enjoying it.
Have you considered that you have joycon drift issues? Because this isn't really a matter of my experience vs yours unless your copy has a different physics engine. If you're not holding the joystick when you land, you will not move forward after a jump. The effect would be more noticeable than in the other games because Mario accellerates more quickly.