Question

Does the ability Iron Fist boost the power of Sky Uppercut?

Answer 1

It boosts all of these moves:

Bullet Punch
Comet Punch
Dizzy Punch
Drain Punch
DynamicPunch
Fire Punch
Focus Punch
Hammer Arm
Ice Punch
Mach Punch
Mega Punch
Meteor Mash
Shadow Punch
Sky Uppercut
ThunderPunch

So yes, Sky uppercut gets a boost.
http://bulbapedia.bulbagarden.net/wiki/Iron_Fist_(ability)

Comments

I have a Hitmonchan.Should I teach it Drain Punch or Sky Uppercut

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Drain Punch, definitely. With such Low HP, it'll be easy to restore it via Drain Punch, especially when paired with Iron Fist. Sky Uppercut isn't as reliable, as well.

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However, that low PP for Drain Punch back in Gen 4 is annoying. If you are too reliant on using a Fighting attack, Sky Uppercut would probably be better.