I can't tell yet. Although they had her systematically tracked, it's still unclear why, I think. It could be simply designed to intimidate her, or to learn something about her to blackmail her into compliance with Trump's scheme, or to find out when/where the best place to murder her is, after learning her schedule and habits.

Edit 2: I was wrong about that, by the way. It turns out doing a yet-unspecified bad thing to Yovanovich was part of a separate quid pro quo with a former Ukrainian prosecutor (Yuri Lutsenko), who was willing to provide "dirt" on Biden in exchange.

Edit: Surely, the US tracks/monitors/spies on foreign diplomats here on American soil, and it's probably warranted, in a lot of cases. But Yovanovich was almost definitely targeted because she was heavily involved in anti-corruption efforts, and was morally unassailable. They knew she'd take issue with and report Trump's pressure campaign.

Christ, I'm so happy that Zelensky was elected. If Poroshenko was re-elected, his administration had already putrified to the point that it was receptive to Trump's demands, and we would have never known that it was Trump tryna dig up dirt on sLeEpY JoE all along.