Presumably POTUS is having a dig at Strozok, who is a clown working for the FBI
Mueller has clearly been involved in some very bad shit over the years. The only question is, did he do it voluntarily, or was it blackmail? Once the ball is rolling and he did his first bad deed, that then becomes part of the overall blackmail package which grows larger over the years.
I'd say he'd done enough bad stuff to be trusted by the cabal which is why he got the job of prosecuting POTUS. I'm hoping he's still a good guy inside and the strings have been cut, allowing him to seek revenge (best served cold).
I know this goes against what Q has said, but I'm leaning towards RR being a black hat through and through and Mueller being a black hat through blackmail who has now flipped. I know, I know - if one is bad, both are bad; if one is good, both are good.
Then again, disinfo is necessary.