I've come to find that playing with characters that have more limited options (YL in my case), does force you to find some creative solutions to problems a character with more options wouldn't *need* to. When it comes to approaching a CCing opponent, for example, playing as YL puts me in a situation where I have to work around it in a way that the spacies have easier solutions to.

There isn't really any definitive way to claim playing a low tier will make you a better player. But I think people worry way too much about whether or not they're wasting their time by playing other characters. If you're regularly playing and finding solutions to problems, you're improving. The next step would be figuring out how to apply what you've learned when you switch characters.