Your drawing isn't too bad, however there isn't really one solution to improving it since everyone has a different style. I can only describe to you how I improved my drawing.
The program you use to draw comics can have a huge effect on how the comics look. Many people will swear by MS Paint, but you could also use GIMP or Photoshop. Personally I use paint.net, it has a similar feel to MS Paint but with a lot more features.
I find that using a mouse makes it more difficult to draw smoother lines. I use a laptop with a touchpad, which makes it far easier to fine tune your drawings. Thicker ball outlines generally make it easier to draw smoother balls, however I think well-drawn thin outlines look great, but that can be difficult and time-consuming.
Adding ball shadows isn't a requirement, and whether you want to add them or not depends on what you want your personal style to look like. If you want to use shadows, paint.net is great because of its layering function. You can draw the ball, make a new layer, set it to be transparent and draw a black blob where you want the shadow.
The comic that you drew doesn't have any major problems in terms of art, but you might want to improve your drawing of the USA. Remember that it has 13 stripes of equal width. It can be quite difficult to get them equal width, but as long as you have 13 stripes (7 red, 6 white) then it shouldn't be a problem.
Hope this helped!
My most recent comic was done on an iPad in procreate, but the one for the rest of the comics was thisTurcom from amazon that was like $50. I know like nothing about drawing tablets so idk if it’s a good one but it works.