Well, that's the point isn't it? To solve a problem within the game's boundaries? If you can tie a chain of events that led to the loss of your stock to a poorly placed fsmash, I don't see the harm in thinking you should have been smarter about your choices. But, if you cannot fathom a reasonable solution, then I can see the issue with this mindset. I suspect that would be the case with characters like Ness and Kirby, and I didn't mean to include the whole cast in this. I'm not advising blind devotion. This is meant to be applied in a situation where a player can pinpoint deficiencies in their choices, where a reasonable solution can be applied.
And btw Strongbad, I think if Mango was actually dedicated to Mario, he'd have similar or better success than Shroomed. And shroomed is a guy who's widely considered top 10, with a character like Doc. Also, I think that has more to do with Mango's playstyle. You can't look at a guy like Mango and say "he never makes mistakes". He makes plenty, and Fox/Falco cushion that a lot better than Jiggs or Mario can.
Actually that gets me thinking, what high level mid-tier main would have better success if they turned into a sheik/fox main? I've never seen it. I think the idea of switching mains to get better results applies more at the lower levels, where character choices can impact the outcome more than skill can. I mean, I can't see a top player blowing everyone's mind by becoming strictly a Falco main. Also I'm aware there are top players that switch characters to make it easier on themselves, that's not what I mean by switching mains. I suspect they'd be quicker to say that they switch because it's too hard, rather than to say a matchup is "impossible". That word is thrown around too often at lower levels.