I often play Roy, so not only do I have a counter but I also have the third best horizontal air speed in the game along with an Up Special that covers good horizontal distance and a really high fast-fall speed.
When I play Pit, my answer to that situation is often just to put up my Guardian Orbitars. Even if the hit breaks them, I still avoided the Smash Attack. Problem is that often leads me to get grabbed and UThrown where I'm then in the exact same situation again, except minus my Orbitars. Other option is to Power of Flight towards the edges of the stage and grab the ledge.
Mii Swordfighter's counter has a delay between blocking the attack and actually countering, allowing the Swordfighter to fall down into the foe and actually hit with the counter (normally, a well-spaced USmash with disjoint, such as Ike's or Link's, will cause the counter to completely fail to hit). Lucario and Greninja also have counters that are useful in that regard.
Link, Toon Link, Samus, PAC-MAN, and Mega Man all have projectiles that go straight down and can easily disrupt it. PAC-MAN's Side Special can also be used to great effect simply by aiming left or right and down to get him to the ground as fast as possible. I'm not sure about Yoshi's Egg Throw, but I think a low-distance, low-height throw can actually go a short distance below Yoshi. Also, Yoshi's Down Special can help mess up their timing. If you do it wrong, though, you'll just run yourself into a slightly less-charged USmash.
Bowser's Bowser Bomb or Down Aerial can also be used, but again, if you time it wrong you'll just run into a slightly less-charged Up Smash.
Edit: And of course Kirby just drops straight on their head with Stone. Unless their Up Smash does more than 20%, in which case at least it blocked the damage for you. Or you can Final Cutter to fly back up over their attack and then land on them.