Did Jerry have some kind of mental condition?

Child like and low IQ don't immediately call for having a mental condition. I think he hasn't developed much from being a teenager. He lived his life like a teenager. But don't forget; he lost his leg when he was 16, that certainly changed his life for good. He was an incredible artist and a person with a good heart, but as he said in the end "we are losers" and it's true. Their upbringing cornered them and they had no idea of how to live a stable life style.