Your movement is pretty good. I'd suggest working on lengthening your punishes, because as a midtier, the struggle is trying to find a good approach and then getting in. So in the Mario-Fox matchup specifically, find habits that your brother commonly does and then exploit them.
General Mario improvement tips:
Movement
Short-hop Up-Air strings (really important)
SHBAWD (a post was recently put up on how to do it)
Up-B Wall Jump
Fast-falling your short hop fairs a little bit better, and then following up with a grab (most of the time if you hit them, you're guaranteed a grab, but if they shield, it's unsafe)
Full hop dair into an aerial, preferrably up-air, second option is bair
Utilizing Mario's Dair (Check out A Rookie's video about this. Extremely helpful!)
Mario vs Fox matchup tips:
Mario has a chaingrab on spacies. From 0%, he can up throw twice, assuming that the second time, there is absolutely no DI. Follow up with an Up-Smash on the second throw, potentially jab reset, and then find another follow up. It's different depending on DI.
Mario has a guaranteed chaingrab from 38%ish to 68%. After you upthrow the fox and he exceeds 68%, a short hop up air fast falled into a forward smash should do the trick if executed properly.
Hope this helped, man. It's hard to critique a fellow main after only watching 5 minutes of gameplay. But I hope this helped both you and others!