There is no choice of words that will make it normal to install mobile apps from anywhere other than an app store. Whatever word we use will take on the meaning of doing something unusual.
"Sideloading" doesn't have an inherent or deeply ingrained negative connotation. I don't see a reason to try to change it.
Let me just "sideload" an app onto my laptop...
Does that make sense at all? "Sideload" and not "install"?