On one hand, differences between two characters can bring limitations to the weaker character's options when facing its opponent, but on the other hand, that doesn't stop the same top level players from placing well in tournaments, even with such varying character choices. I don't think Melee is balanced, but I think that if the match-up permits, even a character with the ****tiest moveset can perform if played right.
It really comes down to the dedication of the player in learning the limitations of your character and making the best of the tools at your disposal. I honestly think that with enough work, if the match up isn't too skewed in one side's favor, a low tier could definitely contend with a high tier, but yeah it's stupid how much work it would take for a low tier compared to, say, a space animal, with so much more depth and a satisfying playstyle.
At the moment, there could be so much more to explore for chars like Yoshi and as ICG defends, Pichu. Honestly, who knows just how viable these characters really are? They've been in classifications ranging from complete **** (Kirby) to incredibly technically demanding for viability (Yoshi) to a mediocre Fox (Pikachu), that an aspiring player will likely be turned off from such absurdities. Why pick up a ****ty Fox when you can be Fox? Why learn tech skill for Yoshi, when starting with Fox would be less demanding and could be applied to the other viable characters too? But there are so many videos, guides, and discussions for the current viable characters, why would anyone go out of their way to learn something new, something that hasn't been found in the last decade?
I'm sure, in some way, Project M can remedy the insane technical demand for the eggsecution of the Yoshi ledge-egg stall thing seen in Melee, but if shining is a "cheap gimmick," then how many cheap gimmicks should Yoshi get to be viable? If P:M is more balanced than Melee, with balance resting on giving characters new tools, doesn't that take away the depth of the gameplay that was found in Melee? I feel like trying to work with the stupid Melee glitches and mechanics, as hard as it can be, to make a character viable, is probably a more rewarding experience. If you add cheap and gimmicky elements to the low tiers and expect them to perform better and be viable, it only can discredit the work, say, a Fox put in to perform at an X level, when it can be achieved by a less-experienced/less-talented player with these shiny, new tricks.