I'm not sure what that's supposed to imply because if Ursula von der Leyen manages to be less corrupt without being elected by the people that has some uncharitable consequences for the quality of the American electorate, I think HL Mencken had something to say on this
Technically, the US president is also not elected by the people but by the electors, which in many ways is comparable to how the EU elected the EU Commission President.
How should EU citizens vote at that next election if they are unhappy with von der Leyen's actions as President of the European Commission in order to remove her if enough other voters agree?
They should vote for parties that are not organized in the EPP (European people's party). The exact party options depend on the country you're voting in.