I'm not overreacting. Anyone who would put such a nasty sticker on their vehicle clearly has no respect for their adult children. If they are willing to broadcast the supposed "selfishness" of their daughter to everyone around them, you know that this sticker is just the tip of the iceberg.
Imagine what they must say to their daughter in private if this is what they think is appropriate to say about her in public.
As someone who does not have kids, this sticker would just anger the hell out of me. Even if I fake joking with them, I would permanently lose respect for my parents knowing that they truly thought this
This 'joke' is not an offhand comment that wasn't thought through. Someone thought this, found and purchased the sticker, installed it, and lives with it every day. I would definitely cut someone out of my life if they continuously disrespected me like this, at least if they continued to do this after I confronted them about it.