Patella Mobility Norms
Is there a standardized objective amount of ...medial and lateral patellar mobility, or do orthopedic therapists pretty much just reference the non painful side? Aka...what is "normal"? 
I don't think there are any standards (if there are I don't know), and using the other side may actually be a very poor choice.In fact, the only legitimate number I was able to find was 5 degrees of patella lateral rotation during flexion of 20-90 degrees.Assuming there no pain, the knee cap's mobility is only of relevance if mobility and/or stability are limited post-injury or if in preparation for a special or specific activity.There is contra-motion of the patella during knee flexion and extension, where the patella must glide south during knee flexion and north during knee extension. If either of those ranges are incomplete or uncontrolled, improving patella glide is likely warranted.But I would be willing to guess that if we compared right vs. left or one knee vs. what we may think is "normal," there will be situations where lower body mechanics are very competent despite some kind of measure of the knee.Patella Baja, Alta, stiffness into medial glide are all hooey if someone can squat, step, and lunge pain free with good form.In fact, where there is evidence is where if you take McConnell Taping strategies, knee pain is positively effected taping the knee in any direction you choose. So medial glide as a measure of incompetence or causation is not accurate. That doesn't mean don't tape; it just means it's not the medial direction that is always the solution.And we can go back to Chris Powers' earlier work from 2003-2007 that showed it was far more femoral rotation creating aberrant knee mechanics than a medial patellar glide.{I had a bunch of other things written here, but I just deleted them. Clamshells and lunatic were among them. You can imagine the rest.}
As for the other side, hyper- or hypo mobility may or may not have been a part of the problem in the first place, so maybe the low tone of the hip that was contributory to the ill mechanics and injury also manifests through low tone in the patella. It would not be useful to reference the uninjured side simply because it may also be showing you increased risk in a non-painful state.So all in all, treating the patella can have great utility when knee motion requires restoration, and/or there is an appreciable gap between active and passive performance. In my experience, mobilizing the patella has been more useful in highly stiff knees post-surgically, but just anything, anything can work for anybody.........., but nothing always works for everybody.