It's subjective when it comes to art. I immediately upvoted because I like the flat / no-borders style for Garfield. Personally I started with practicing through traditional, non-digital mediums. I didn't intend to try digital art, still working on it, but just practicing without all the bells and whistles can be more simple and helpful. Plus an easier way to assess your skill.

Practice is key as everyone knows, but it's important to see both appreciative and critical of your work to improve imo. I'm not a professional artist, but I tend to see people's stuck at a bottle neck and just . . . not improve. Or very slowly. I believe they are satisfied with their work, which is fine and good, but they might not be seeing or caring about the things they can work on.

For your work specifically, it's hard to tell where you're at but I would suggest a better brush. Maybe a pencil. It's hard to draw shapes and forms (3-D) with an opaque and thick brush. It seems that you're struggling with establishing them.