Like, you can be skeptical of the idea that the Russian interference was decisive in the election without dismissing the very real lawbreaking that happened.
I think it was inappropriate for him to stress this, and that it undermined his legitimacy and the legitimacy of the investigation. It isn't the job of an investigator to "exonerate" people that havn't been charged or prosecuted for a crime.
As far as I can tell all these comments did was validate his supporters beliefs that the investigation was a politically motivated attack, and simultaneously served as a psychological "out" for his detractors that were convinced he was in cahoots with Putin to undermine the country. Widening the divide between 2 sets of people that really ought to reconcile.
Disclaimer: I am not american, so this is an outsiders perspective.
Trump's supporters would have taken any action beyond cowed silence as evidence of a "politically motivated attack," because that's been the president's messaging. The investigation itself was characterized as "politically motivated messaging," despite having been started under Trump by Trump appointees.
Mueller isn't responsible for "widening the divide," and there was nothing he could do to heal it while living up to his mandate. The president, or at least the president's team, did a lot of illegal things during the election. You don't meaningfully hold them to account or heal "the divide" by falsely exonerating the president.