That's a good and bad comparison. It works on the level of "switching to a top tier character because you want to win", but it also doesn't work because MK is substantially more broken then marth, or anyone else in melee. MK is god of brawl, no discussion. Marth, or anybody else in the top tier, have their counterpicks and weaknesses, they are not indestructible. (Yes some might try to make the argument that sheik is god of melee, but if that were true then tournament results would match those of MK in brawl, and they obviously don't.) Picking one of them up is not "taking the easy way out" but instead dramatically improving your chances of success, and fun. Winning is fun, and honestly, using a high/top tier character is quite fun. Switching to MK would be taking the easy way out, while switching to Marth would be like picking up a legitimate competitive character.
Because relying solely on one character is unwise because:
1. It might cause some of your play to become stale. I learned alot about spacing, and how to do it appropriately, when I finally picked up Jiggz and Ganon. If you only use mewtwo, your abilities and skillset might stagnate, derailing your improvement.
2. It will completely hinder you in competitive play. In competitive play, when somebody sees you pick up mewtwo, they will probably select one of his absolute worst matchups (Marth, Ganon, Sheik, or Captain Falcon). In this scenario, it will be good to have a good counterpick to any one of those characters, you would want to be skilled with either, say, Jigglypuff, Sheik, or Fox, in order to counter the opponent's character that destroys Mewtwo.
3. Picking up a high/top tier is fun. Using a good character is just plain fun, it's like finding a really good weapon in some shooting game and just blowing away your opponents, it's just really **** fun.
No. Honestly, that will never happen. Mewtwo is THAT bad. Mewtwo will never be a LEGITIMATE threat to anyone who knows the matchup and how to exploit mewtwo's many, glaring, if not immediately subtle, weaknesses.
If anyone actually has ever played a good mewtwo, and knows what to look for, and how to respond appropriately, your chances of winning are pathetically low. It's for this reason that you do want to pick up a high/top tier character: when they figure out your mewtwo's tricks, you better have a legitimate response.