You'll be seeing a lot of luigis out there, so it's important to know the matchup. Personally, I think its 6:4 luigi's favor. I'll start with my favorite set of this matchup that I've seen:
With that, here's a bullet point list of important points in this matchup:
-Luigi can shoot fireballs faster than you, but you can outcamp him still thanks to the fact you can bounce your fireballs. Abuse the hell out of this. Forcing luigi to approach is always a good thing.
-In the neutral, his fair can beat most of your options, making it quite frustrating to win a footsie war with him.
-Your only guaranteed combos really are dthrow into upb, nair into jab combo, and dair into followup. True uptilt chains are out of the question, and upair strings are extremely risky. Any bonus combos you get outside of those I listed are going to come from reads/conditioning him not to throw out nairs. You can sometimes get one uptilt off after a dthrow too before an Upb.
-On the defensive end, Luigis nair is a much better escape move then our upb since its a lot safer. Mario can upb out of most luigi combos that aren't downthrow->fair or downthrow->downb, but Luigi gets a way harder punish for reading our upb than we do reading his nair.
-Off stage, mario wins. Luigi's recovery is good distance wise, but mario can punish it during the side b or down b phase of it. All of your edgeguard tools are pretty effective (Fair is used fantastically in the video I posted).
-FLUDDing him always helps onstage or off since his main weakness is mobility; Toss in fludds to get him where you want.
-The biggest disadvantage for Mario in this matchup is his lack of guaranteed kill setups on Luigi, while Luigi has a guaranteed one, and its extremely easy to get. I'm pretty sure its nigh impossible for Mario to di out of Luigi's down b, even if they screw up. I know I'm doing the DI right because I get out with my other characters when the Luigi mistimes their mashes, but not with Mario. As for getting the grab, one wrong shield will get you grabbed thanks to +1 frame advantage Luigi fireballs <.<
The TL:DR of why Mario loses 6-4:
1. Luigi Fair is fantastic in neutral vs Mario.
2. Luigi Nair is a much safer escape option vs Mario than Mario's upb is vs luigi. Nair also gets him out of more combos than upb does for us.
3. Luigi has an easy to land guaranteed kill setup; Mario does not.
TL:DR of Mario Pros:
1.Can out camp luigi
2.Luigi is vulnerable to pretty much all of Mario's edgeguarding tools
3.FLUDD is fantastic in this matchup, both offstage and on.