I think it’s a valid thing to pay attention to, and your example supports that, doesn’t contradict it. In the early part of the 2016 election, Clinton was considered a shoe in against Trump. But Trump excited his base h he filled stadiums - while the support for Clinton was much less enthusiastic. And, as we know all too well, Clinton lost.

So yes, seeing the kinds of crowds a candidate draws is one measure that’s worth looking at. It’s not the whole thing, for sure, especially when some candidates aren’t above paying people to attend.