There is a trending hashtag moderation system, where the moderators review hashtags before they appear in trends and can block them before they are allowed to appear.
However, as far as I can tell there is no way to moderate or filter non-trending hashtag suggestions.
I've tried to find a relevant issue about this on the Mastodon github, and following it up on there:
https://github.com/mastodon/mastodon/issues/26454
Sorry I can't help more, this is a really good point you raise!