Peter III was defintely more German than Russian. Left alone in Germany, he would have been fine, I think that's correct to state. But when he was autocrat of a huge, unruly Empire, that was a far cry from a tiny German state where he could have been eccentric had he wished, and no one would have cared. He wasn't mentally retarded, he just was backward a bit, and had some strange ideas. His childhood wasn't a good one, and that sometimes scars people. But the best source we have on him is Catherine's memoirs, and I would be sure she didn't want him thought of well, not that he would have been. He was way out of his element in Russia.