Maybe it’s genuine hakaras hatov?!?
If the the mayor did nothing wrong, there is no need to come on to the reason of 'genuine hakaras hatov' in order to justify such a letter.
However, if the mayor did commit an anti-semitic act, which is the obvious case here, there is "zero" justification to be choinef a rasha because of past/future favors.
Bottom line, hakaras hatov is irrelevant.