It is OK to change a baby diaper in public.
However when you change a baby diaper in public you should be considerate of others around you and also when possible give the baby some privacy.
Many people believe that just because they are a baby it's okay to change the baby wherever they are but that is not the case.
While people understand that a baby needs a diaper change they don't always want to be around the baby being changed.
Nor does everyone want to see your naked baby while they are being changed.
Even though the baby cannot be embarrassed you should still give the baby privacy when changing their diaper as there could be pedophiles out there looking and possibly taking pictures of your naked baby being diaper changed.
I've changed my babies and toddlers in public when needed but I would try using something to cover them up with such as a towel, blanket etc.
Or if possible I would change them in a bathroom stall or in the car, van etc.
As the baby gets to toddler age and reaches 2 years old and above they require more privacy.
Don't simply pull down a 2 or 3 year olds pants in public and change them in plain view of everyone because 2 and 3 year olds can feel shame.
Take them somewhere private to change them.