The spikes in Clinton’s graph result from the difference between being in office and running for office. Whenever she ran for office, her ranking plummeted (probably because of the attacks from right wing media seeking to stop her — “The Stop Hillary Express” Sean Hannity called his campaign). When she was in office, her ratings were generally favorable (more so than Trump’s).

In my view the reason Clinton’s ratings haven’t gone up again since 2016 is that she’s still fighting the 2016 election, and her platform comes across as sour grapes, not hope for change.