I suspect that Trump would approve of any mail-in vote that is for him. He says that absentee ballots, like he and Pence and a lot of other White House staff use, are somehow good, but other vote by mail methods are bad. But an absentee ballot is indistinguishable from a mail-in ballot; that’s exactly what it is.
If, as he claims, going to vote by mail will prevent any Republican from winning an election, how does Cathy McMorris Rodgers keep doing it?
Of course, Trump and his supporters have never been bothered by such minor issues as the truth of their statements. A lie, oft-repeated, becomes fact.