The problem with Versailles is not that it was too "harsh" -- it was after all, well within limits established by Germany itself in its treatment of defeated Belgium and Russia.
The problem was that Germans did not feel defeated militarily, and so did not think they deserved any punishment for the war.
Nor did average Germans even know their own national leadership had started the war in the first place.
They believed the "war guilt" portion of Versailles was totally unjust.
Germans had hoped they would be treated by President Wilson according to his "peace without victory" promise and "14 Points".
When that didn't happen, it was a major grievance.
Nevertheless, as the German economy slowly recovered during the 1920s, Hitler's appeal was very minor.
Only with the coming Great Depression in 1930 did Hitler's words begin to gain traction amongst Germans.