Firing her was a campaign promise that Johnson made to voters during the campaign. I guess people are now saying that he should be scolded for doing what he said he was going to do? Most of the people whining about this are people who were doing Vallas victory laps the day before the election.
Besides, it's perfectly normal for a new administration to replace key positions with their own. That's why these people are called "political appointees". Again it's crazy that Johnson is for some reason expected to keep the people around who are 1) opposed to his agenda and 2) partially responsible for the last mayor losing her election.