Everyone knows one is stock and the other is flow. It doesn't mean that we can't measuring the ratio between them. Actually, measuring ratios between "stock" and "flow" values is one of the favorite things analysts and economists do! (e.g., rent vs house price, P/E, fixed cost vs marginal cost...)

There is even a name for marketcap/GDP, Buffett Indicator. And historically the current value is very high.

[0]: https://en.wikipedia.org/wiki/Buffett_indicator