Yes in competitive play, no elsewhere.
When something is on the line, you are going to use every tool at your disposal to try and win. That includes getting good with a character that is considered viable.
However, when it comes to For Glory lagfests, item FFAs, and single-player? It really doesn't mater. Play as who you are most comfortable with, or stick with whatever characters you are attached to even outside of Smash.
I play as Rosalina most of the time, regardless of context, but it isn't because of the fact she's one of the best characters in the game. It's because I just really like her as a character and she was my most wanted newcomer for Smash. And I love how she can be played in all manner of playstyles, and still remain really great. You like to play defensively? Rosalina's great. You like to play offensively? Rosalina's great. Like to perform lots of mix-ups? Rosalina's great. Etc, etc.