Clinton would probably lose in 2020, even if she handled COVID well or even mildly competently.
By TTL 2020, Democrats will have held the White House for three terms and Clinton probably would have been facing unrelenting GOP opposition that would have made her legislative agenda effectively DOA (even if the changed presidential race allows the Senate to flip in 2016, the House will almost certainly going to remain Republican).
Even butterflying COVID away, she'd likely have few accomplishments, an incredibly-energized opposition and the Democratic party apparatus having been hollowed out after years of neglect from Obama (and likely Clinton) and three consecutive midterms delivering absolutely brutal results against the party in Congress and for state offices.
Also, Trump in all likelihood is not the nominee in a scenario where Clinton wins in 2016.
If he loses in 2016, all of the "never Trump" Republicans, or even those like Paul Ryan & Mitch McConnell who IOTL were obviously hedging their bets until the election results came in, would have been validated in his inability to win a general election. Without the aura of being a winner (a product of decades of self-promotion,
The Apprentice, bodying more established candidates in the GOP primary and upsetting Clinton IOTL 2016), Trump doesn't gain the ability or prestige within his party to reshape the GOP to his will.
What happens to Trump? Probably what it's rumored he was planning to do if he had lost in 2016--use the attention and support he gained from the campaign as a jumping board for a TV network (
Jared Kushner met with potential investors to discuss the idea in October 2016 IOTL) and continue claiming the 2016 election was stolen from him to get more attention/dollars.
Who is the Republican nominee? I'm not sure. The GOP's habit of nominating the primary runner up in the next election cycle would mean it would be Ted Cruz, but Trump's clowning of all the 2016 candidates would seemingly eliminate the possibility for any of them to run again in a party like the Republicans, whose base voters clearly want a strongman-type who can bully others into submission as their leader.