What will also help u not get walled out by fireballs is short hopping over them towards your opponent and coming down with a well spaced Nair. This way you are gaining stage control and pressuring him/her. If your opponent starts shielding them Short hop over them into a grab or cross up Nair.
Takes some practice, but not that hard. Try to mix up your approach game and don't stay on the ground so much. Incorporate more aerial approaches in your game. Pressure the Mario with Nairs, tilts, jabs. Space him out and force mistakes.