Hitler was a mediocre painter who never had a chance. His perspective was strong, but he never developed the ability to capture the human form and he was profoundly inflexible. Many have pointed out his dismissal of abstraction, but in the interwar period many artists were returning to classicism (see Picasso's neoclassical period from 1917-1925). What kept Hitler from achieving success in art was Hitler. He didn't need any help being an arrogant little shit.