As long as you aren't theatrically loud so that you are disturbing the guy at the corner table writing his thesis, I wouldn't give a flip what other people thought. Your child will only want you to read aloud for a very short while, and then they'll have mastered the skill and prefer to read alone. I read to my kids in doctor's offices, parks, airports ...
Think of this - for your entire life as a mother, others will be looking at you or your child sometimes a bit askew because of their differences. While we don't want to raise boastful, disdaining children who have a superiority complex, we also don't want our kids to not be ok with who they are.
And for Mrs. Public who interrupts mid-sentence, don't look up. Just smile and hold up a one-moment finger and continue reading to a stopping point. That is what I used to do.
