Quite likely that even if Mueller were more forceful, the GOP still wouldn't convict.
However, it might have put more pressure on the republicans to actually deal with Russian election interference. Perhaps caused a few GOP members to decide that tying themselves to Trump might be problematic. Maybe even cause them to lose some voter support.
Instead, allowing Barr and Trump to control the messaging around the report (and thus more or less sweep it under the rug) meant that there was no reason for the republicans to clean up their act.
(I do think there were other things Mueller did wrong... allowing Trump to avoid testifying, not going after the Trump hell-spawn.)
