They put in serious effort to mock Trump, obviously.
They didn't dare to mock Obama or Biden.
Tom Shillue does Biden — rather well. SNL has also had Biden impersonators — who weren’t very good. Late night talk show hosts all support Biden, and even if they did still book impressionists, they don’t want Joe being mocked, any more than they would tolerate jokes at Obama’s expense.
Impressions aren’t as big a thing as they were 40 years ago, or even 20 years ago. Some people do Trump. Some did Bush. Key and Peele had an Obama routine. But nobody is famous for being a copy cat in the way that Rich Little or Frank Gorshin were. Dana Carvey, Billy Crystal, and Darrell Hammond have all moved on. Phil Hartman has moved a lot further on.
They put in serious effort to mock Trump, obviously.
They didn't dare to mock Obama or Biden.