It's a sign of a long period of stock market increases, for sure. There are many such signs, such as the increasing prevalence (I must find a way to quantify this but I think it's true) of "Why not 100% stocks?" and "20 years to retirement, why bonds?" postings. To say nothing about enthusiasm for cryptocurrencies (when everything is going well and people are feeling flush, they start to feel that they can afford to gamble a bit of it because they're playing with house money now). "Wealth effect."
Of course, you don't need signs that the stock market has been doing well for a long time, you can just look at the stock market itself. And in both cases, it is possible to feel uneasy without there being actionable signs of a top.
The speculative types talk about an asset "going parabolic" (and argue about whether to try to jump in and try go get some of it--sure it's a bubble but there's plenty of meat left on the bone). It doesn't look like that to me yet. Of course I'm retired and wouldn't hear any stories of people quitting their job in order to "become day-traders."