Since Imposter works as an automatic Transform, which copies the opponent's exact stats other than HP, does it matter whether or not I use an Imposter Ditto with a certain Nature, IVs and EVs? Well, I know the HP isn't copied, so I know one should focus on getting a good HP IV and max its HP EVs, but what about the rest?
Imposter Ditto's non-HP stats almost never matter. Almost. Transform can fail in certain rare cases:
Other than that, Ditto's nature, IVs, EVs and stats are irrelevant, except for HP. It automatically transforms as soon as it switches in, assuming the target's stats, and using the target's IVs for the purpose of Hidden Power.
While there are moves or abilities that can strip his Imposter ability (e.g. Mummy), none of them can affect Ditto until after he's already transformed.