AFAIK, there are no "native" SNMP plugins for monitoring mail queues, of any kind, not just Postfix.
However, there are lots of plugins and scripts for monitoring mail queues in such a way that MRTG can monitor, and even graph, the data.
And, you can always write your own scripts for querying the mail queues for stats, and returning the data in a faux MIB to bsnmpd, which would then return the value to the SNMP client. See the "external" section of the config file.