First off, mario's u-air hurts. It can juggle,and it racks up damage insanely quickly. Similarly, his b-air is also a pain. Although it isn't as good for juggling, he may use it to approach. As far as his ground game goes, he'll use U-tilt and D-throw to start some u-air combos on you, and stutter-stepped F-smash has a fairly long range and can kill. His jab and U-smash are decent but nothing too special, whilst his D-smash hits both sides quickly and sends you out at a pretty low angle. His fireballs are an okay projectile, but they shouldn't be a huge problem.
Don't forget mario's cape. Spamming T-jolts probably isn't a good idea, unless you're doing them from a platform above him. Also, if you're going to thunder, make sure it's either too quickly for him to cape, or that the bolt won't hit you should he cape it.
Edgeguarding mario is possible, but somewhat tricky, as although his recovery is somewhat limited, he can use the cape to stall for time. Mario can edgeguard you better then most other characters can thanks to his cape, but you should be able to avoid it. However, be careful recovering from below, as then our recovery loses its unpredictability, making it much easier for him to cape. Obviously, skullbashing offstage is certain death if he's anywhere near you.
So, to recap...
Stay on the ground/below him.
Be careful using T-jolts/thunder
Don't recover from below.