claws-mail is based on GTK instead of Qt. While in theory both speak wayland to communicate with the compositor, in practice the Jolla compositor only implements a old set of wayland protocols. GTK does not support these but rather only a different set of old protocols and a few modern ones. Implementing/extending something like QtRunner to support newer protocols is likely involved.
Then you would need to package the whole GTK stack, lots of tiny packages.
Finally I don’t know if claws-mail uses libhandy or similar to implement a reactive interface or if you would need a magnifier/scrollbars.
There are ways to run Ubuntu or similar in a container with a window containing a separate desktop but I don’t think that would feel very sexy