Broadly speaking there are two strains of denialism when it comes to the Muller investigation: Trump supporters and some left-wing and libertarian activists. You’ll see some overlap in the sorts of accusations from both camps, but the Trump supporters stick to pretty simple insult and gross distortions of facts. There’s not much of an actual argument to be found.

The left-wingers (if you can call them that) seem to argue along these lines:

1. Anti-Russia sentiment is getting whipped up in the media and anti-Trump people badly wanted to believe Russia was behind Trump’s election. This desire was behind the investigation and the focus on Russian interferance: There was no actual substance to the claims. The Muller report didn’t support their beliefs so they should give it up. Read the Report
2. Actually the report details a lot of questionable behavior and finds the Russian government had attempted to influence the election through the use of social media and hacking, whether or not the Trump campaign’s cooperation rises to the level of conspiracy. But we can’t believe anything in the report with regard to Russian interferance; that information in the report came from the intelligence community which we all know can’t be believed about anything. Read the Report
3. Even if they did try to influence the election and did hack the DNC and release emails it probably didn’t change the outcome so it doesn’t matter.
4. If the Russians did change the outcome it was at least good we learned about corruption in the DNC.
5. What’s wrong with messing in other country’s elections? The U.S. does it so it’s only fair it gets done to them in turn.

You could categorize this series of points “defense in depth.” There’s always a fall-back position.

They fail to address the whole reason for the investigation: Trump behaved publicly like he was trying to stop any investagation into Russia’s role in getting him elected. He did this in many well documented meetings at the White House, Twitter posts and T.V. appearances and through his actions as President.

To be clear there were two investigations: The first began in 2016 when evidence surfaced of Russia’s eforts to get involved in the elections, and this continued through the fall of 2016 and after the election.

The second began after the firing of the director of the F.B.I. (in charge of the existing investigation) in the spring of 2017 and Trump publically stated he was thinking about the current investigation into Russian election interferance when he fired the director. That same week he met with the Russian embassador in the White House and told him he had put the Russia investigation to rest by his firing of the F.B.I. director.

No counter-intelligence organization, having public evidence such as this, could ignore it and do nothing without making themselves a joke. Even if they suspected Trump was only behaving in such a bizzarre manner because he felt the existing investigation made him look less of a winner, rather than that he truly had something to hide, they had to be seen to look into it to preserve credibility if nothing else.