XDR is not directly comparable to this as it requires pre-agreed structures.
ASN.1 is slightly more complex in this regard: BER can be mostly understood without knowledge of it's intended structure but only mostly (actually, MessagePack seems to me like BER done right(-er)), PER requires schema in any case. Bust most significantly ASN.1 is mostly about how to encode the schema itself, which is completely outside of the scope of MessagePack.
There seems to be a pretty fatal flaw in MessagePack that doesn't exist in [BCD]ER. MessagePack doesn't explicitly represent strings and only provides a binary data type.
Besides not being able to distinguish between a true binary blob and a string, the two parties need to agree on what string encoding should be used. Is it UTF-8, UCS-2, EBCDIC? I suspect this would create tons of incompatibilities between implementations as various parties make their own naive assumptions about what strings are encoded with.
That seems like a pretty major flaw to me. X.690 is a bit overly complicated (there are 10+ string types...) but there is such a thing as too simple.
ASN.1 is slightly more complex in this regard: BER can be mostly understood without knowledge of it's intended structure but only mostly (actually, MessagePack seems to me like BER done right(-er)), PER requires schema in any case. Bust most significantly ASN.1 is mostly about how to encode the schema itself, which is completely outside of the scope of MessagePack.