For Mario, it's best to use your basic Neutral tools. Ridley somewhat heavy has a really big hurtbox. This helps Mario's up-air strings connect, so it's usually better to choose stages with platforms. Use Mario's tools such as his grab, d-tilt, u-tilt, and short-hop Nair. Nair is Mario's quickest and usually best OoS (Out of Shield) option. Be wary of Ridley's Neutral-B and Side-B, by including short-hops and full-hops as well as dash-dancing. Play safe and stay aware when Ridley whiffs an attack or a grab to then punish with and aireal or a grab of your own.
Mario