I've said it a million times. vs Luigi is even.
Mario has an edge camping, and to a lesser extent edgeguarding, while Luigi has an edge KOing ridiculously early if he can land one of his short ranged KO moves, and then some of his combos do slightly more damage.
Luigi's N-air is very manageable if you know what to expect. Just U-tilt whenever he's above you. It will always beat his N-air or D-air. And I do in fact mean it when I say "always".
The hardest thing to deal with in this matchup is just Tornado, since he will generally use it to punish your ending lag on stuff. Fireballs can stop it in its tracks, and if you predict it, F-smash can beat it.
Mentioning F-smash, Mario's F-smash is so ridiculously good in this matchup, it very much makes up for Luigi's crazy KO power by being a solid counter to a lot of his moveset and physics. F-smash is safe on block in this matchup, and it outranges Luigi's entire movepool. This allows you to consistently mindgame the **** out of him for some clutch KOs in this matchup. If Mario's F-smash KOed just a little earlier, this matchup would swing to Mario's favor. It's that good in this matchup.
In short, both combo the heck out of each other, and while Luigi may have some dirty tricks up his sleeve, Mario can F-smash him in the nuts to put him back in his place.