I used the reviews. Sometimes a movie would be polarizing and I’d read the good ones and the bad ones to see if I’d like it myself. It’s not a hugely valuable feature (I can do the same on eg Rotten Tomatoes), but it wasn’t unused.
70% of Netflix's viewing happens on TV. You couldn't even access user reviews on tv.
15% of Netflix's viewing happens on smart phones and tablets. You couldn't even access user reviews on mobile.
So 85% of Netflix's viewers (TV, phones, tablets) couldn't even access the reviews from their viewing-devices.
You could only access reviews on desktop/laptop... Which accounts for only 15% of Netflix's viewers.
In a lot of ways, the user reviews are mostly a vestige of when people used them while adding DVDs to their queue.
Talking about a popup is a non sequitur, it's not like the web site does that.