That's a bunch of bullshit. The left is married to the idea that Trump somehow cheated his way into the White House, and anything short of impeachment, will see the same reaction you saw to the election results. There will be scores of idiots on the street protesting and screaming "Cover up!", and "Still not my President!", because like the election, they have been sold a lie.
Mueller will be vilified as incompetent, and corrupt, and all the right wing talking points on him now, will become left wing talking points.
Guaranteed.
No, they just believe it's a waste of time, and has been used to overreach. The investigation is hardly even about collusion anymore. It's just one big snoop.