Please do provide evidence of this. Most people I've seen being surveyed in Europe around tourists don't care where they come from, as long as they spend money and don't ruin stuff.
Americans, along with the British and the French, seems to be the groups that most people complained about when it comes to groups vacationing in Europe.
Edit: lot's of people replying, which is great. But none of providing evidence, which is explicitly requested in this comment. Would love to see some hard facts rather than people's anecdotes.