Pit will focus on zoning Mario out, since Mario is a combo fiend. Mario's fireball is far superior to Pit's Arrows though, so feel free to dart in and out while throwing fireballs to force Pit to approach or do something that's otherwise punishable.
Pit is a Punish character, so he'll be looking for you to mispace yourself, throw out a laggy attack, or being predictable with your landing habits. Stay mobile and unpredictable, and be sure to punish him if he whiffs any dash attacks or grabs. His Pivot Grab has deceptively large range.
One you get inside and get a grab, you can go to town on Pit, since he's just the right size to be combo'd. Watch out for his Frame 4 Nair that is sometimes used to combobreak. It can catch you off guard and break your combos, but it has a huge amount of landing lag as a price, so shield it and punish it with almost anything you want. If they jump out of the classic Dthrow > Utilt strings, you have the aerial mobility to follow them with an aerial, mostly Uair. See, Pit has 3 mid-air jumps, but his aerial speed is among the lowest in the game to compensate. And to boot, his jumps don't give him much height.
tl;dr You want to wait for your change to get inside, and then do as much damage to Pit as possible. Don't let him get away, because he really doesn't have the mobility to stay away from Mario once Mario gets inside, and that gives him a chance to use the space the relieve some pressure. Pit does NOT do well while under pressure.