Not the first time I’ve seen this theory, going back to the original appointment. There was an actual Congressman or two claiming it last week as well.
I’ve seem other people state that Mueller was illegally appointed because he was supposed to investigate collusion, which is not a crime. Special counsels are only supposed to investigate actual crimes.
In the end, Muller is a tool of the DC Swamp, which is trying to expel Trump from the Body-Politic for not being one of them.