Luke seems his optimistic self even after he gets his face chewed up by one of Ziggy Ackbar's band members, but once he meets Yoda, he is pissed off pretty much the rest of the time he is with him--and ONLY with him.
Once he is away from Yoda, he reverts back to his old self.
Was he racist against Yoda?
I think this explains it.
He didn't like little green people of color.
He didn't want to eat his food.
He wouldn't eat his own food after this "person of color" sampled it.
When he was given advice not to take a weapon, he took it anyway.
When he was told he could lift the ship, he didnt accept it.
And then when he sees the person of color raise the ship, he doesn't believe it.
Maybe Darth Vader really told Luke that Yoda was his father and that was why he was so upset.
I kind of can see a resemblance between older Luke and Yoda...