For kicks on the way down from a shorthop, just practice a bit in training mode and it shouldn't be too bad.
If you mean on the way up, it's got to be done immediately as you jump, rolling from jump to attack. Tested it out against a few characters:
Metaknight: Just could not get the sweetspot kick on the way up. Could graze him inconsistently, so it might be possible at great pains.
Olimar: Could get a sweetspot off on the way up, but not consistently. Would take a lot of practice, and even then it would be hard to get off in an actual game.
Mario: Could get a sweetspot consistently, though wouldn't try it unless you're certain.
Link and above: Seemed easy to do and consistent.
Actually getting in the position where you can use it safely is the hard part... Lag online might affect that too, though the timing on the jump and attack shouldn't change.