The accent's not exactly fake.
Ahmed Best, the actor who played Jar Jar (i.e., wore the
Jar Jar suit and got digitally adjusted), is really from the
Carribean.
George Lucas had originally planned to have a
voiceover actor read the lines, but Best ended up doing it himself. I believe that
actors' union rules would then stipulate that he got paid about a bazillion times more for the role than if he'd just worn a rubber suit, jumped around, and let somebody else read.
Not that this is any excuse for the existence of Jar Jar. At all.