Just because you are playing only one character doesn't mean you aren't improving at the game. On the contrary, learning the game with a single character helps you learn the game better because you don't need to spend time learning new character's kits. You know all the stuff Maokai can do that other characters can't, so you can easily use Maokai to learn all the stuff every character can do and needs to do in order for you to succeed at the game.
As a side note, MOBAs are actually unique as a genre where the majority of the playerbase plays at least 2-3+ characters, whereas in most other competitive genres the majority of the playerbase will use at most 1-2 characters (at least at a competitive level) because it takes so much effort to learn a new character and it takes so much time away from practicing your current main. Playing one character almost exclusively doesn't quite have tradeoffs that significant, but you still get the benefits of being really good at Maokai.