Due to the way that the effective permissions are computed - through things like derived roles etc - it is tricky (but not impossible) to produce a list of permissions for a role as the result will be:
• only under some set of condition
If you take for example rules which match based on time of day, you will get back a different set of permissions depending when the request is made.
Generally we see just the roles being exposed in a UI to end users, and the nitty gritty Role<>Permissions logic kept just in the policies.
That said, with the Admin API you can fetch the policies currently being served from a Cerbos PDP and then derive this yourself into a UI.