I don't believe that it is. I follow this "scene" pretty closely, and that means I read about successful scams all the time. They happen in huge numbers. Yet I have never encountered a reliable report of one that utilized a "sideloaded"[1] malicious app. Not once. Phishing email messages and web sites, sure. This change will not help counter those, though.
I don't even see what you could accomplish with a malicious app that you couldn't otherwise. I would certainly be interested to hear of any real world cases demonstrating the danger.
[1] When I was a kid, this was called "installing."