i feel that the Kilo Kilo fruit could have been used much better. But i wouldn't say that it's the most retarded fruit out there.
If any is to be called the most retarded devil fruit, then that Spring leggy fruit would have to take the cake. I mean, it's practially just a branch of Luffy's Gomu Gomu fruit. And it would only apply to the legs, right? It is a major downgrade, and even Usopp would be able to kick that guy's ass.