Micro.blog can be used without a Mastodon account if your desire for “Mastodon” is:
- People can follow your blog on Mastodon.
- People can reply to your blog on Mastodon.
- You can reply to those people from Micro.blog
- You can follow people’s Mastodon accounts.
- You can reply to people who are on Mastodon.
If the feature set of Micro.blog as a social platform matches how you want to interact with social platforms, then the ActivityPub interoperability covered by Micro.blog is all you need.
If you want to have a fully native experience of the Mastodon service, you’re better off using a Mastodon account and cross posting (in addition to making your feed available, IMO).