There is no standard format for the plain text part of bounce that your MUA shows to you as it's just that: plaintext part to show when MUA knows nothing better to do. RFC 3464 specifies format of additional MIME parts of message that specify what happened in machine readable format. It's interesting to note that it's exactly same mechanism as is used for sender-requested notifications for delivered email in which case these messages are processed by MUAs just fine.