JMAP is not really meant to be used with desktop client apps, rather it is designed to work with webclients instead.
However, we did investigate the possibility of supporting it, and even created a prototype eM Client application. Unfortunately the only real implementation of JMAP is Cyrus, which is full of bugs and incomplete features, so not really usable.
At this time we don’t see any benefit using JMAP instead of IMAP, but if it goes through a rewrite and has some actual server support, we may revisit it in the future.