After being blungeoned to death by numerous For Glory Little Macs in search for a weakness for Little Mac, I can finally report I've found one, and it's almost the same as Ike's.
Now normally, if you attack an Ike in one of his attack states, your attacks gets overridden by his more powerful sword (so in trade-off terms a bad deal), but if you use a grab more often in place of an A attack, the grab uses the open window to interrupt his attack and always wins out, making Ike vulnerable. Due to his slowness, he's also vulnerable to light jabs.
Well, it turns out, after analysing so many games... Little Mac has the same weakness, but for a different reason, but with the same outcome. Normally when Little Mac punches versus say, a smash move, because of super armour, he doesn't flinch (and this goes for light jabs etc) making him almost unassailable.
But as you might have noticed, he has the same flaw as Ike: that powerful punching doesn't apply to grabs. Grabs interrupt an attack fully, regardless of super armour or not. And because of the hitstun on toss, he then becomes vulnerable to frisbees, cans etc. So Little Mac's weakness is... grabs!
And as an added bonus... it nullifies slip counter.
However, unlike Ike, he's immune to light jabs in most cases, and his speed presents a problem. So unlike Ike, his weakness is to grabs and projectiles (where-as Ike is weak to grabs and light jabs). Use projectiles to punish his retreat or distance, and then when he gets close, don't bother jabbing or smashing, go for grabs and projectile again. Eventually his percentage will increase and he'll start to juggle. Be sure to toss him off the edge so his air game is on edge.
So my friends, use grabs more heavily and in-place of A attacks with Little Mac. And if he spot dodges, frisbee punish.