I seem to have a habit of picking "bad characters" for my mains.
Yet despite this, I always seem to have way more fun playing these allegedly bad characters than I do top tiers, and I find myself winning more consistently, even with the lower tiers (except for Brawl Ganondorf).
The point is, if you are willing to put in the time and effort to a character that you are passionate about, you will likely see some results.
Will you be taking tourney's left and right? Unless your main is Sheik, than no. But you will be enjoying the game more and the losses won't seem as frustrating. And believe me, when you choose a "low" or even "mid tier" main and start going competitive, you will lose. A lot. But if you keep going, and push your character to extremes, you'll be surprised with what they can do, especially in Sm4sh.
Kind of as a side note, tier lists don't matter all. Match up charts do. Luigi is (or at least was) considered a high tier character, but despite this, he has bad matchups with many low tier characters, including Shulk. Based on a tier list though, you would never see this, and many luigi players expecting to dominate a Shulk of equal skill may find themselves for a rude awakening. For another example, Ganondorf, who is often seen as the worst heavy character, actually has a slightly favorable matchup against DK who is often considered the best heavy character. Again, a tier list would not show this, and its for this reason why if you want to choose a character, look at their matchups, and determine if the unfavorable matchups matter more to you than the favorable ones.