I obviously don’t know what this is about, except being a religious occasion.
It is a belief in Judaism to do a mitzvah (a commandment of G-d) as nicely as it can be done, within reason.
A nice dress seems the right idea, from a Jewish perspective.
I think it is what is “within reason” that is being discussed. In my opinion, that’s a personal choice. However, the social media aspect of it seems intended to emphasize the conspicuous consumption, which would not be the case if the child simply wore the dress to the ceremony.
I wouldn’t know a $1,000 dress from a $100 dress from a great find at the thrift store.