When a Client Identifier is dereferenced, the resource MUST be serialized as an application/ld+json document unless content negotiation requires a different outcome.
Why does Client configuration data have to be JSON-LD serialized?
OpenID Federation Specification defines Automatic Registration feature that achieves the same goal and the Entity Statement is a signed JWT: https://openid.net/specs/openid-connect-federation-1_0.html#section-10.1