As a Mario main, I should be able to help you.
He can't deal with disjoints, and his lack of ability to deal with disjoints is his biggest weakness. Because he's got a stocky body, he can't get in that easily unless he utilizes shield pressure. Since Mario is excellent at applying pressure to shield, staying back and being *just barely* out of his range will help you avoid his solid combos. For me, my least favorite characters to play against are Shulk, Palutena, Marth, and Lucina. Those four have great range and are tough for Mario to deal with. If you can outrange Mario in general, then great. However, those four characters are able to do it the best.