No — manual approval is off by default. The app is designed to keep the path from sign-up to promotion as short as possible, so new ambassadors get their code immediately.
If someone signs up who you'd rather not work with, you can delete them in the Ambassadors tab.
Some merchants prefer not to share their portal link publicly at all, and only send it to people they've already vetted. That gets you most of the way there with no extra friction.
If you do want to review everyone first, you can turn approval on: How to require approval for ambassadors. Two things to keep in mind — requiring approval can noticeably reduce how many ambassadors finish signing up, and Reject permanently deletes the applicant's account with no way to undo it. If you might want someone later, leave them pending instead.