Fun idea, and very funny execution.
I also made a drawing game and one issue that I noticed here that I also ran into is that if you move your mouse fast when drawing, Godot only registers a few of those events. So to make the drawing a lot smoother you could add all the pixels between each event and the previous event. I don't know if that makes sense, but it would improve the drawing experience by a lot. It made a big difference to my game.
Anyway, good job. It would be interesting to hear how the grading algorithm works!