For a Mouse work this is really good. The next step for you would be upgrading your gear to a tablet. There are lots on the market for a good price and everything with pressure sensitivity is better than a mouse.
I feel that you have some very nice ideas and potential so don't let your gear hold you back!
As for now you can start with cleaning up your black lines. It has some charme to have multiple lines - if they have purpose!
Example: Santas coat. Makes it more rough and like its been through some stuff.
Bad example: Santas face. Get those helper lines out of the face! It just makes your picture look "not done yet"
Looking forward to some more art from you!