For a 401(k) Roth (Designated Roth) contributions it is not necessary to report anything on your 2020 tax return. The earnings will be reportable on your 2021 tax return.
However, your question is confusing since you said "mega-backdoor Roth". Was the excess actually to a Traditional 401(k) prior to the conversion? That would be a different answer and require entering the returned 401(k) contribution as taxable income on your 2020 tax return, but not the earnings that are taxable in 2021.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
