The only reason Aipom/Ambpom should have Nasty Plot is to Baton Pass it. Aipom's base Sp.Att. is 40, and Ambipom's is 60... you'll pretty much have to max out your Sp.Att. with Nasty Plot to be able to do significant damage, and they don't have the defenses to last those three turns of stat boosting. Also, Jolly and Adamant reduce your Sp.Att. even more. You'll want Timid/Modest if you go through with this set. If you're going to evolve it, you'll want Run Away instead of Pick Up since Run Away become Technician, and Technician boosts Swift.
But really, a Special Aipom should be a Baton Passer along the lines of:
Nature: Timid
EVs: 252 HP/6 SpAtk/252 Spd (the goal is to survive long enough to pull off a Nasty Plot or two and then BP them, not to stay and fight)
-Nasty Plot
-Baton Pass
-Thunderbolt/Thunder
-Substitute
With that lone Special Attack serving little purpose in-game, but used to counter Skarmory that may try to PHaze you in competitive.
