I realize this is a good problem to have, but still, there are too many posts in my bookmarks. I am a Mastodon hoarder 
@Em0nM4stodonEm
Same here, I would like to be able to put Masto bookmarks in subfolders like I do with my browser bookmarks.
If you have a fediverse account, you can quote this note from your own instance. Search https://mstdn.social/users/MostlyTato/statuses/115264447208962307 on your instance and quote it. (Note that quoting is not supported in Mastodon.)