Hashtags, used appropriately and in moderation are absolutely appropriate on Nostr and useful for discovery.

You can do anything you want on Nostr, and no one can stop you from using hashtags inappropriately, though. Hashtag spam is definitely an issue here. Little will get you muted faster than using hashtags to make your unrelated content more visible. So, keep that in mind when deciding to add them to your notes.

My general advice would be to only add a maximum of three or four highly relevant hashtags to your note. Bonus points if you can fit them inline within the context of your note, rather than placing them at the bottom or top of the note. If you have to choose top or bottom, choose the bottom of the note so people can read what you have to say first, and disregard the hashtags.