Wildcard certs aren't yet supported by the ACME protocol. There has been discussion on the IETF ACME mailing list (search link), as well as issue 97 at the original acme-spec github (Note that repo is mostly for historical purposes now that the IETF process has begun).
The conclusion was to tackle this in the working group / mailing list:
I agree [...] that this is a difficult thing that the WG should discuss.