Maybe the Left does it just as much, but my impression is that the Right tends to dismiss actions based on perceived intent. For example, Mueller's actions are biased, so his indictments are without merit. He has an "agenda" so his charges are null and void. He is ignoring Hillary, so that's unfair, dismissing his actions against Trump aides. Manafort is being forced to testify against the admin., so it can't be the truth. And Trump's favorite, "they are just upset about losing".
So why is it a valid argument to say, if your intent is suspect, your findings are dismissed? First, I posit that this is because of the "bad seed" narrative we are taught that originates in the bible. In this scenario, the world is divided into good and bad people. If you are a bad person, whatever you promote is not to be believed. 
Now the Right does not think they are perfect, because they understand the importance of humility. They are better than to think they are better than other people.