I fail to see where Romney did anything wrong.
He bought near the peak of the market (OK, his timing was certainly wrong), and then when his property value went down he went through the established process to lower his assessment to a more correct amount. It appears he did it more than once as the market kept falling.
Is that ethically wrong? Is that economically irrational? Of course not. If he had not corrected his assessment in line with a falling market, he would be stupid. I don’t want a stupid guy as president.