Well, in terms of competitive breeding, (which has you breed individual values or IV's to increase stat gain by leveling up) you usually leave the stat they don't need blank. The stat choices are usually between Attack and Sp. Attack. You're reasoning is actually pretty good for someone who's new to it, though I do recommend giving Mimikyu the modest nature. Modest nature boots Sp. Attack while lowering Attack. This is recommend because most of the moves Mimikyu learns are special attack moves, hence why attack isn't really necessary. In the end it's up to how you'd like your Mimikyu to preform so it's totally up to you!