It depends. I wouldn't bug someone with a FF toddler.
I did mention to a woman with whom I was riding in an elevator that her baby's straps were too loose and the chest clip should be moved. Actually, I started out by simply fawning over the baby to see how receptive she was to talking to a total stranger about her child. Once it became clear that she was happy to talk to me at all, I mentioned I was a car seat tech and that, while it was perfectly fine strolling around in the mall, I hoped she would tighten the straps up a bit before she got back into the car. That led to questions about chest clip placement and it turns out the mom was also worried that she didn't really know how tight the straps should be (so I got to demonstrate the pinch test for her) and she said she was having trouble tightening it up over the fleece suit... so we got to have a whole conversation about how to safely keep a LO warm in the car seat and so on, and she seemed really appreciative and left knowing more than she did before we crossed paths.
Basically, I won't approach strangers with an "I ran you down so I could criticize your car seat usage" situation, but if I have an opportunity to bring something up, I will do so as long as the parent appears receptive. I also hand out my card which has the URL for the local calendar of check-up events (as well as my email address).