What's the exact behavior of the installer? It doesn't use the same code at all, so that's purely a coincidence it it suffers from the same problem.
Don't worry about the importance, though: I'm currently discussing with an adduser developer, and the fact that I set it to Low doesn't mean it won't be considered. Though if you say the installer is affected, we may want to raise priority - but that will be more important when we know what the fix is, and when backporting will have to be considered.
Personnally, I think chown's behavior regarding the dot is silly, since ':' already serves the same purpose (maybe that's a legacy). Anyway, you can always quote the username if you want, and users-admin will ensure chown correctly interprets a username with a dot in it. So I'd rather fix adduser.