The description contains the mime/media type of the response. But the description is not necessarily exhaustive, nor is that media type necessarily the default one the service returns.
It is possible that a description contains application/json, but the default response type of the service is application/xml, thus derailing the whole deserialization.
Kiota should read the response media type from the description and include it in the generated code. If multiple media types are present, the should be appended according to the RFC: application/json, application/xml etc...