I've never played the match up but I play Little Mac and Luigi. Here's what I think:
This is a very tough match up for Luigi. Mac out ranges him with his footsie game and Luigi's slow air speed makes him vulnerable when jumping. The best method is to wait him out. Fireball from a distance but don't spam it. Mac can side b and smack you in the face if you constantly throw fireballs. Give yourself a chance to shield if he goes for it (most likely he will because Little Macs don't jump). He might also decide to inch in and shield every fireball. If he does this follow up your fireball with a grab if he gets to close. If he rolls past your fireballs you can catch him with a cyclone to catch his rolling.
If Mac is in your face I would suggest returning the game to neutral as soon as possible. He can overwhelm Luigi and you have few, if any, offensive options against him that close. Most likely Mac will pressure with spaced Ftilts. If he spams you can roll behind him between Ftilts and grab to start your Luigi stuff. If he's pressuring too close then shield grab. If he connects with a Dtilt you can surprise him with a cyclone to quickly come back down and attack. Or you can fast fall a Nair. Be sure to mix up between offensive and defensive options.
Like with any Mac match up, getting him in the air is key. Take advantage of frame traps. Make him double jump and/or air dodge and attack when the moment presents itself. Uair is pretty much always safe. A properly spaced Uair on Mac's aerial Counter will make him miss. When he's offstage he has some options but not many. Above the ledge, you have to read when he'll use side b and counter with a Bair. At ledge height, he has to side b so be ready with a Dsmash or Bair. When he's below the ledge, don't stand there waiting for him. His up b has a large hitbox and will hit you if you're too close. You can drop down and get him if you think he'll make it back.
Recovery for Luigi is very difficult. He can swat away side b recoveries. He can Dsmash anything that recovers low. Luigi's up b moves very quickly and it's hard to time with a Dsmash. If he can time it well then there's much you can except delay up b the best you can using a combination or double jump, side b, and rising cyclone to throw off his timing. And SWEETSPOT it!!!
I think this is
60 /
40