This is where the game of politics gets sticky.
As others have affirmed, most ambassadors have robust and not entirely complimentary views of the country with which they liaise - the difficulty occurs when, as in this case, such opinions are leaked to the wrong people.
The ambassador's views were clearly not meant to be heard by the US government, or the President, but they were, with predictable consequences.
The British government have been forced to back their man in order not to look like they can be bullied by the US, and fortunately, further damage is averted by his resignation, so everyone can calm down.
If President Trump is going to continue in his role, it's way past time that he realised that not everyone thinks he is as utterly fabulous as he thinks he is.