For me it would definitely depend on the style and pattern, the onesies that just look like a normal t-shirt I would wear in public with either shorts or pants. As long as you have pants on they usually just look like you have your shirt tucked in.
I have over 300 solid color onesies, either its my overshirt or undershirt, i have every style from hawaiann/wife beater to golf/polo shirt (A golf shirt onesie with a sportcoat is as dressy as i get ) my diapers are always supported and ride were they do the best job, the business end is always in my pants and without anyone ever figuring it out .
I did not know LittleLoliKat modeled for the ABDL community (She's on YouTube). You can get away with wearing any onesie in public, just wear pants over them and people will think you're wearing bizarre shirt. I wear one to work every day under my uniform. On the way home I take my shirt off and nobody knows what I'm actually wearing.