There seems to be some confusion here.
In order to be a double negative in the sense that you mean, the two negatives must be in the same clause.
independent clause; negative used to underscore absence of an as-of-yet-unidentified claimQuote:
I have never claimed
optional introductory word for dependent clause, which I omittedQuote:
that
dependent clause that admittedly could do without "pretty"; internal litotes ("not bad"), clause as a whole refers to the claim the independent clause said I wasn't makingQuote:
every other job in the game is not pretty bad compared to MNK
The two negatives create the effect of a litotes throughout the phrase ("I wouldn't say it isn't bad", if that serves as a simpler example)
Dig?
EDIT: "I didn't do nothing" could actually be a good example of a litotes, though not in the sense you're using it. "I didn't do nothing" could mean something distinct from "I did something" in the same way "It isn't terrible" means something distinct from "It's great".