I was really hoping for a GraphQL API standard for mail before reading this. But this sounds good too.
You could have a variant of JMAP with GraphQL syntax and semantics, but there would be a fair bit of mismatch, for their purposes and focuses are quite different: JMAP is concerned with object identity and synchronisation, for what you might call thick clients (and JMAP Mail needs to be broadly IMAP compatible), while GraphQL is UI-focused, for what might reasonably be considered to be thin clients (not that they are logicless, but that they are very much focused on offloading burden to the server). I see no compelling case for a JMAP-like GraphQL thing.
It could still be an interesting task to develop a object synchronisation protocol like the JMAP core on top of GraphQL.