Do you think it would be polite of the parent changing the diapers to ask those nearby if they minded, first? That would be a form of politeness. Often there is room elsewhere on the plane, far more roomier than a flight seat, to accomplish the task of changing a diaper.
As a former Delta Flight Attendant - I have helped mothers change diapers on my jump seat - in the back of an empty row of seats far removed from other passengers - and (with blankets) on the floor of the galley area. There is no good place to change a diaper on an airplane - unfortunately....