Mueller's farewell speech was designed to accomplish the same objective as his report; imply that Trump did criminal things even though Mueller found no evidence of criminal activity by Trump.
Mueller spent time trying to make people think about Trump when he was talking about Russia's attacks on the electoral process even though his report said there was no collusion what's so ever. If there was shred of decency in Mueller he wouldn't have done that.
Mueller also tried to make it appear that the only reason that he didn't indict Trump was because he's a sitting President but that's a lie.
If, as Mueller concluded, there was no collusion then it would be impossible for Trump to have committed a crime related to collusion hence there was no basis for Trump to be indicted on any charges related to collusion.
As to obstruction Mueller himself, as Barr has pointed out, failed to point to anything that he could show was obstruction. Mueller listed a few things that "might" have been obstruction but prosecutors don't deal with "might". They say that something is either criminal or not; they may be overruled by judges or juries but prosecutors don't just randomly tell the public that something someone did "might" be wrong. They don't do so because it's not their job to smear someone's reputation it's their job to prosecute crimes.
Given that among the things Mueller pointed to as obstruction was Trump wanting to fire Mueller and replace him with a less compromised individual--a key person in the obstruction story was Comey who is Mueller's friend-- it's clear that Mueller was trying to fabricate something. Unless Mueller thinks that he's the only person who could have run this investigation replacing him with someone else and not stopping the investigation can't be obstruction.
If you look at the things that Mueller claimed "might" be obstruction you'll find that none of them are
things that sane people think of as obstruction. Contrary to what Mueller tried to imply in his speech none of the items mentioned in his report involve lying to or asking someone else to lie under oath. But they do include Trump saying that someone who was attacking him was a rat.
Mueller made a big deal of not being able to indict a sitting President but he failed to mention that his report did not say that any action by Trump was obstruction. Rather he listed a few things that "might"be obstruction. It is a matter of fact that Mueller now or in his report did not say that but for his inability to indict a sitting President he would have indicted Trump or even that he had evidence of any crime. Mueller's report didn't say "there is evidence of obstruction but we can't indict"; it said these things "might" be obstruction.
He's trying to avoid directly lying while doing his best to imply that despite his, Mueller's, inability to find any evidence of any crime that Trump did in fact do something criminal.
One way to see the dishonesty of Mueller's position on obstruction is that it's impossible to obstruct a witch hunt and Mueller's admission, which he tried to cover over in his attack speech, that there was no collusion means that there was no valid investigation to obstruct. Essentially Mueller is trying to criminalize a wrongfully accused individual fighting back against an unjust politically motivated investigation.
In light of that it's amazingly dishonest for Mueller to have
said this in his attack:
"And beyond department policy, we were guided by principles of fairness. It would be unfair to potentially — it would be unfair to potentially accuse somebody of a crime when there can be no court resolution of the actual charge."
Some of you may be thinking that calling Mueller a fascist is a bridge too far. But here's what he said:
"And as set forth in the report, after that investigation, if we had had confidence that the president clearly did not commit a crime, we would have said so. We did not, however, make a determination as to whether the president did commit a crime."
In our democracy we are innocent until proven guilty. In fascist regimes people are guilty until prove innocent.
Mueller's comment that they didn't clear the President is saying that he believes Trump is guilty until proven innocent.
The way our system works is the following. If there is evidence of a crime, which there wasn't in the case of Mueller's investigation of Trump, a prosecutor investigates. If sufficient evidence is discovered then the prosecutor indicts and brings the person to trial. If there is not sufficient evidence then the prosecutor does nothing. He certainly doesn't write a report and make speeches about how he didn't prove the alleged perpetrator to be innocent precisely because doing so is, as Mueller said himself, unfair.
Yet Mueller's report and his speech do precisely what he says is unfair; imply that Trump is guilty of something while not in fact presenting any evidence of any crime.
There are two options. Either Mueller is an incompetent who, despite spending a fortune, didn't find evidence that exists or he's a dishonest political hack who in spite of finding no evidence is doing all he can to malign the President he personally doesn't like.