The Mastodon convention is that they refuse to provide comment notifications without a mention in the comment. It is a silly requirement which isn't part of the ActivityPub specification, but people will complain bitterly if they don't get notified about your comments. What I've been doing is adding mentions of the author of the thread parent and also anybody that he/she mentioned. It *should* be enough to add the tags to the ActivityPub object without requiring visible mentions in the note itself, but I'm not positive Mastodon supports this. I know Pleroma used to provide these as "hidden mentions" but now they are all visible and I suspect it's because Mastodon requires them to be visible - I haven't checked personally.
