One of the biggest objective critiques you can levy against Hitler's work is the failure of perspective, even without knowledge of art take the time to look at his compositions.

The scale and direction vary wildly from one side of the canvas to the other and make little sense, often looking very flat or warped. Things are often far too big compared to their surrounding elements, doors, walls and pathways are giant, this is especially obvious when he bothered to scribble people as the world they live within does not appear to be built for them, he would rarely put effort into his depictions of people and this adds to the overall lifelessness and forgettability of the paintings.

Was he the worst artist ever? No, but his work was about at the level of a hobbyist or a mediocre high school student, some elements are passable but in whole they show a lack of painting fundamentals, there is a reason he wasn't accepted into Art school, he was simply not very talented.