This reminds me of an episode of the podcast Weird Work where they interview someone who works as a bridesmaid for hire. For a fee, she'll help with wedding planning and go as far as flying out to act the part of bridesmaid in your wedding [1].

So this isn't just a Japanese thing. Just as I think that most people in the US would find hiring a bridesmaid in the US to be a little bit weird, I'm sure most people in Japan would feel the same way about the service described here.

[1] https://bridesmaidforhire.com/