Is Google Maps's "Popular Times" feature only for restaurants, or is there a way to enable/view it for non-restaurants?
The Popular Times feature is available for any business for which Google has sufficient data to generate the graph. I can see popular times for a swimming pool, a zoo and a shopping centre for example with a quick search.
The Google My Business Help page for the feature has the following to say:
Popular times, wait times, and visit duration are shown for your business if it gets enough visits from these users. You can’t manually add this information to your location, and it appears only if Google has sufficient visit data for your business.
As a user there isn't a way to toggle this feature on or off for a business.