Little Mac has ruined the entire game for me. Online is a joke when everyone is just spamming Little Mac. I'm incredibly disappointed that Nintendo even allowed a character like this to exist. Every other character is fine but Little Mac is the most broken character in the game and playing vs him all day is boring. Oh well- guess I'll just play some offline with friends and move onto a different game for online fun.
Howdy there, Mac main here. The problem with those using the people's champ is that there's only two extremes; the Smash newcomers that are using him because he's a relatively high-reward character with lots of powerful moves and seemingly few weaknesses, and the far more dedicated players that chose Mac because of these weaknesses. Because of this huge split in the overall player skill of people using him, you'll really only need to learn how to deal with both ends of this spectrum.
For your guys that chose Mac because he's fast and hard-hitting, a thing that works wonders against them is the fact that you can bait out a few select moves really easily. Create some distance between your character and their's, and I can guarantee that almost 9 times out of 10 that they'll blindly run at you and either use a dash attack or the Jolt Haymaker. As long as you keep your shield up, you can grab a Mac during the cooldown on his dash attack and just throw them over the ledge to get them in the air. If you can get a Mac player offstage, they've only got two ways of recovering and neither of which is particularly good. If they sprint at you and throw out a Jolt Haymaker, with a bit of luck they would've triggered it too late and will fly straight over the ledge. No hassle! Another thing to watch out for are his smash attacks, which, due to their super armour frames, make them a fantastic choice to use over and over and over again. There's no real trick for dealing with Mac's smash attacks -- just stay calm and play a bit more defensive than you may normally do, and sneak in a few grabs and throws when you get the chance.
Dealing with players that are actively maining Mac despite these huge faults is a lot different. Typically speaking, we're playing some mental Punch-Out in our heads, constantly cycling between styles of patience and punishment.
A skilled Mac player who's playing patience is trying to bait or trick you into approaching them, or forcing you to make distance that they can then exploit. Something I've noticed in my own play and the play of others that use Mac when playing patience is that we absolutely love to shield in our own approaches. There's nothing stopping a patient Mac from dashing up to you and raising their shield to try and bait out an attack, and likewise there's nothing stopping them from dashing in, shielding, then dodging back to pull you to the middle of the stage. A patient Mac can be a pain to deal with, but it's not a style that can let them win matches, so expect them to default into this sort of style at the very beginning, and in the windup to a stock being lost and the calm of a player returning to the fight with zero percent. Once again, just try to keep calm and remember this style of play isn't gonna let a Mac player win.
On the other hand, when a Mac player goes into punishment mode, the best thing to do is probably to go equally all-out. He's a powerhouse, sure, but he's still very reliant on getting up close and staying on the ground. Simply out-moving him by getting him into the air and playing near the ledge is a huge deterrent to playing a punishment Mac, not to a mention a huge blow to the overall effectiveness and sheer size of his moveset. The biggest thing with fighting this 'mode' of the guy is that intimidation is a great way of making it look like he has fantastic range, when really he's got one of the shortest reaches out of every character. If you're playing someone like Samus, Fox, or Toon Link, projectiles are your friend; they'll simultaneously keep him at bay while forcing him to close in... And, of course, once he's in, he suffers from the same overall problems of range and reliance on select moves. Expect to see lots of forward tilts, down tilts, jabs, and upward-angled forward smash attacks when they get in they close, with the occasional up smash thrown in.
I know this was a pretty needlessly long post, but I feel like Mac really isn't getting enough attention in terms of information. The people using him either don't know how to use him, or know far too much about using him and will try to exploit that you might not know. He's kinda bothersome, sure, but his weaknesses are pretty glaring and easy to overcome with some practice. Hopefully this has helped to some extent, lol.