In order to remove Yovanovitch from her post as ambassador to Ukraine, Trump had his lawyers (aka Giuliani) bash her first. (Quote from NPR: "Giuliani, Trump's personal lawyer, and his associates had begun a smear campaign against Marie Yovanovitch, then-U.S. ambassador to Ukraine.")
It is the full right of the president to put anyone in the position of ambassador or remove them. What would be the point in making Yovanovitch look bad first? Trump fired plenty of people - why would he need to specifically create an excuse before getting rid of her?