99.99% of people in public won't say anything. They're either too busy themselves, or just don't notice. The .01% of people who say something are just assholes who need to mind their own business, and need to be told as such. Clerks can lose their jobs over unprofessional comments, and if you are made to feel uncomfortable, do by all means demand to speak to the manager, and don't leave until you are satisfied. They see people who buy condoms, hemmoroid ointment, KY personal lubricant, tampons, toenail fungus treatment, and jock itch spray. Trust me, to them, you are just another of the hundreds of transactions they will tender during the day. They're too busy worrying about their next smoke break, or when their boss is going to jump their ass next. You buying adult diapers isn't the strangest thing they'll see that day, guaranteed.
Think about it. They have to keep restocking those shelves with diapers on a regular basis, so you're obviously not the only person in your area that uses diapers.
If you see someone you know, most of the time they don't notice what you have in your buggy or hands unless you make some horrid attempt to flee or hide the diapers from them. If they ask, you can tell them whatever you want, truth or fiction, or it's none of their business. (You could always ask them how their sex life is going if they push it.) That usually shuts them up.
Usually the only people who say anything are small children who are too innocent to know your motivation for buying diapers. Their comments are innocent, so take no offense. A good parent should be in control of their child. To sooth the situatuin, you can just smile and walk on by.
In short, no one gives a damn.