"One-hit wonder" means that this creator has released only one, or very few videos at least, that have gone viral - but the vast majority of their videos reach a very small portion of their followers.
It happens quite often on TikTok that creators build a very large following with a single video. Videos that are posted afterwards rarely reach any of the followers they have built up and go unnoticed.
This effect may not be easily recognized at first glance, as the number of followers could lead one to mistakenly assume that this is a creator with a large reach.