I feel like Little Mac's best aerials depend on how well you use them. I personally feel like up air and forward air have the most potential due to the weak knockback and little landing lag that could follow up with other options.
For up air, it could be a mixup for your landing option because of the little landing lag with a 5 frame start up when you auto-cancel it. It might be difficult to hit against shorter characters because the hitboxes are on the arm and Little Mac's head, but it can definitely combo into other moves like up smash, or up B because of the weak knockback. Not entirely sure if it can combo with KO punch, since I feel like a better option would be dtilt -> KO Punch. It can also be used after a down tilt if the opponent tends to double jump as a nervous reaction/quick reaction to escape out of dtilt punishes, assuming you can read this.
For forward air, you can use it at mid percents to force your opponent to land if they're grounded because the move has an angle that sends you slightly downward, which can be used to follow up with strings like fair -> d-tilt/smash attack/dash attack/jab. Offstage, it can be used as an option to follow up with Jolt Haymaker, although it is risky to do (fair -> side B), or as an easy option to gimp characters with poor recoveries like Ganondorf.
I didn't include neutral air because I feel like it has limited uses since it's bad to use offensively, but it can be utilized to interrupt early % combos since his nair comes out frame 2, and it can also be used if you want to avoid using your limited landing options out of hitstun.