For Mario, Snake has a lot of projectiles that'll get in your way. His grenades, C4, and Nikita, and very hard to avoid. Use your normal neutral tools like his grab, d-tilt, u-tilt, and short hop nairs. Nair is Mario's quickest and usually best OoS (Out of Shield) option. Snake is somewhat heavy and has a longer hurtbox. It's easier to connect Mario's up-air strings on him. There is no need to use side-b, as most of his projectiles can't be reflected efficiently anyway. Just short-hop or full-hop over them and under them. Neutral is a pretty great place for Snake so keep your distance with fireballs then approach with a nair or back air.
Mario