Just a curious question to see if anyone has any insights into this
Does your domain name actually affect mobile pop ROI?
on Adplexity I see a lot of landing pages with the domain quick-antivirus-app.com or install-this-now, etc
I would say this is relative and divides opinions. Some guys may tell you that yes it does, and others that it doesn't matter. I would say that it can affect a fraction of the users for sure - sometimes the smallest aspects can indeed make a difference, a lot like angles in creatives for example.
Considering that we're playing in a very competitive game, you should play all the cards you have at your disposal to have that slight edge that could turn the tables on your favour. So yeah, everything that could make a positive impact you should use, especially if it's something relatively simple to do like setting a domain name.
Cheers!
Please consider this, if you put google in domain name, you got a higher chance to be flagged as deceptive website
The impact of a domain name is more significant when working with desktop traffic, since the full url is easily visible in desktop browsers. With mobile, it's not so hot, that's why you can use even a CDN with their default url structure - without using any custom domain at all.
But, this changes a bit when you are using exit or entry alerts, that open those small system notifications : "do you want to leave this site ..." as those display the url in the notification window - in this case, it makes sense to use a domain name.
I'm usually not paying much attention to domain usage, but
Just get a random domain and create a subdomain on it.
The space on the mobile screen is limited, so ppl will anyway not see the rest of the domain.
Here is my tutorial on how you can get unlimited subdomains instantly.
I've noticed that it has more of an impact in specific geo's than in others.
English speaking geo's you need to be more minful.
Some other geos, you could have a domain like... immaSlapYou.com and they would convert very profitable 
Don't worry about it too much, Eriks suggestion is great to start out with. Its not a huge impact trick so dont' focus on it now.
Andrew