When I started playing
, my two other mains were
and
. And I think it can help any Mario player to play, secondary or even main one of those two characters.
Doc helps you learn how to recover safely, as well as make you learn a defensive playstyle, that you can re-use as Mario. It helps you zone with the pills, and having more precision since Doc needs to be precise. In short, Doc learns you how to be defensive, a good thing to learn for Mario.
Luigi on the other hand helps you learn combos and not getting hit. Luigi is pretty light/floaty, so it makes you learn how to not take a lot of damage, to not getting killed early. He also helps for the recovery even more, though he has a much safer one compared to Doc.
All and all, those 2 characters can help you progress your Mario if you play them. I think it's good to main those two if you want to improve your Mario overall.
But that's just my opinion. Feel free to disagree.