That is a fallacy of division. You don't know for sure why Trump appointed him, and it's clear that Trump doesn't always appoint those that will "tow (sic) his line." A good example of an appointment that didn't is Wray as head of the FBI. For all we know he was appointed simply because he'd already been in that position for something like 6+ years so he would have experience.
It's also abundantly clear that Trump didn't appoint him to get Hunter Biden specifically. On the whole, you are simply projecting your own hate of Trump onto this appointment.