I use them in different situations.
If I play a Mario that is actually really really really good, I select the classic yellow overalls just for the classic rivalry matchup.
If I play a guy online that mains Mario but plays like a coward/disrespectful prick, I select Mario Overalls as the Final Diss and destroy him.
If I play anybody a few matches in a row and they piss me off by winning via luck or they get too cocky, I bring out the Cotton Candy as a super diss. I never use the green, just not a fan.
Or if I am practicing with Ganon, Duck Hunt, or Ludwig, and somebody barely beats me in those matches, I bring out Cotton Candy Wario to make a statement and put them in their place.
I like the Red/Pink Biker as a change up.
For tournaments though, it's pretty much classic yellow overalls or nothing.