The short answer: NO.
Everyone else will tell you optins WORK so you should use one; and they do work, if you have a clean and organized site with a strong optin offer.
But most authors don’t – they have disorganized, ugly websites and before you can figure out where to go and what to do, you get a popup that says “Join for updates” and you’re thinking,
Join who? For what? Why should I?
If people aren’t joining your list, it’s not because you don’t have a popup. It’s because you don’t make it super easy to do with a bold call to action, or your offer isn’t good enough, or you aren’t providing social proof or value.
If you’ve done all of that and people are signing up – then you may be able to increase conversions with a popup, however I recommend you pay for one of the premium popup plugins and get something that has “exit intent.”
The means it only shows up when readers drag the mouse up to the top left to go back, or close your site, or go somewhere else. At that point, you’ve got nothing to lose, so a well-time popup could work, if you give them what they want and it solves their problem or they can’t resist.
Don’t have popups “pop” too early – give them 30 seconds or a minute or so to look around. (There are studies for the perfect timing of this but I think it depends on your site).
Check out the popup on Jason Werbeloff’s site, it’s well done: it shows the book cover, some reviews and a sign up button (I would add a screen shot but since I already saw it, it probably won’t display again for me for 10 days or a month or something).