No he isn't discrediting the claims about Russia and by extension Mueller's investigation, he's discrediting the power Mueller has to do anything about it.
People are welcome to hate Maher, I definitely find things to strongly disagree with him about (Israel), but on this, he sort of has a good point. So Mueller subpoenas Trump, Trump tells Mueller where to go, then the issue goes to the Supreme Court, which has been as partisan as the Congress since Bush v Gore, and we assume they make a ruling against Trump, and Trump tells even them to go to hell. Then what? We expect enough Republicans to magically grow a moral fabric to give the Senate the 67 votes it needs to impeach?
I have as much blind optimism about the world being rid of President Trump earlier than 2024 as anyone else, his existence makes the world a morally bankrupt place, but sometimes the realists have to step to the fore and remind everyone what the situation really is.