I think going up against Reagan, who had enormous popular support due to being thought to have presided over a strong economy and progress in foreign policy vs. the USSR, did most of the work in sinking Mondale's campaign.
Reagan wasn't popular by default. He was popular because he was a master at debate, charming people, and framing himself as President.
For example, Jimmy Carter never looked Presidential. For one, he encouraged people to call him "Jimmy" rather than "James". For another, he'd wear a sweater when giving speeches to the public.
People liked that Reagan wore a sharp suit and acted (yes, acted) the role of President.
BTW, Obama, Trump, and Biden all were very careful to present themselves while campaigning as Presidential. They all wore sharp, well tailored suits, and took pains to stand up straight. I bet they all got coaching in body language.
US presidents’ suits tend to be generously cut (if not a full “sack” suit) rather than very well tailored. Viewed in this light Trumps suits seem to have been flattering if unstylish.
BTW, Reagan was a master at reframing difficult questions into a joke, thereby disarming their payload. I remember the Democrats at the time complaining in bitter frustration at how adroitly the "Teflon President" did this.
Not to take away from the joke, but it was a televised debate and Reagan was responding to a question from the moderator, one he probably had an inkling was coming -- not an "argument" from Mondale.
Like I said, I was young at the time. Still, one would expect any candidate to offer multiple arguments for voters to consider. If this simple joke really "sank Mondale's campaign" as claimed above, one would wonder just what form that alleged campaign might have taken. It could be that other factors led to the 49-state drubbing, which factors would have decided the matter even in the absence of a particular canned response to a softball from the moderator.
It's not just my opinion that that joke was the turning point. Although Mondale was clearly outmatched by Reagan, that incident pretty much encapsulated why.
Why are these stories so important to us? Upthread we learn that this debate was as staged as any of Hillary's. Anything planned in advance can't have been an actual turning point. USA popular discourse was shockingly dumb in the 1980s, so it's possible that Jodie Foster had already clinched Reagan's reelection a couple of months after his inauguration.
"I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent's youth and inexperience"
The reframe sank Mondale's campaign.