It depends what you mean by 'natural'. Your sentence sounds like something you would read in poetry, or a Shakespearean play. If that was your intention, then it works, and it would fit in perfectly in that context. If you said it in a conversation, however, you would sound very odd. Also, I don't think 'mere' really adds anything. If it was 'mere animals', that would work, because calling humans 'animals' is derogatory, but 'beings' isn't sufficiently derogatory such that 'mere' has any purpose.

Humans are mere mortals who vacillate between hope and despair.