President Trump’s mistake was not firing everyone (including Comey) during his first day in office. (Well, asking for their resignation, and if he didn’t receive it, then firing them.) Trump repeated GW Bush’s mistake from 2005. If you fire a few hundred people as part of a general housecleaning, no particular case will get much attention; firing Comey a couple of months later, though, got Trump a special prosecutor.
He should never have allowed that. Sessions should have been commanded to unrecuse and fire Mueller.