A first in US bureaucracy

[Labster]

It is unheard-of for a bureaucrat to say something like this about a political appointee. Usually the damage that a political appointee does is worked around, not brought out into the open for everyone to see.

And the letter was signed by bureaucrats on both sides of the political divide.

I doubt this is true.  Both Republicans and Democrats may have signed it, sure, but the political divide is Trump himself.  I doubt you'll find one pro-Trump signature on that list.  This is simply the work of deep state actors, citizens who think that Trump works for them and not the other way around.