SUMMARY STEPS TO REPRODUCE 1. Go to the SDDM theme chooser in settings 2. Browse themes OBSERVED RESULT There are many very amateur themes in the chooser. While there is the banner warning about it being user generated content, so many of these are so terrible that I don't believe plasma should be offering them via the default UI. Wallpaper is one thing, but the login screen needs to be robust and to be competent - a user who stumbles there way into changing the sddm theme and then forgets what they did (it doesn't, after all, reflect immediately) is then "stuck" with an amateur screen. (Furthermore some of these actually crash sddm, per my sibling bug). I'd suggest rethinking whether to even offer the feature (separate ticket), but at minimum there needs to be some kind of qa or gatekeeping to what gets admitted to the chooser.
Personally I agree with you 100% here, but believe it or not, the themes there aren't actually controlled or curated by KDE. They come from a 3rd-party site called pling.com, which mirrors it to store.kde.org. Yes, this is confusing. However the bottom line is that any curation done there can't actually be done by KDE--at least not in an organized way. Rather, anyone who wants to sign up to do this needs to contact the pling.com folks via their website and ask to be made a moderator. In theory any KDE person can do this, and in fact I am one such moderator. However I simply don't have the time to exercise those privileges. There just aren't enough hours in the day to do that in addition to everything else I do for KDE. Some kind of organized effort to sign interested KDE people up for this is probably warranted. However that's not something that can be tracked in the bug tracker as it's not a bug or a feature request. I'd recommend seeing https://community.kde.org/Get_Involved#Contacting_The_Community for avenues to get involved and propose something like that.
And FYI it's a more general challenge than just SDDM themes. A lot of the content on pling.com (mirrored to store.kde.org) is really bad. The moderation needs are immense.