SNMPD not binding correctly

You may want to double check that nothing in /etc/default/snmpd is over-riding your configuration. I think by default, the local loopback address (127.0.0.1) is passed in the SNMPDOPTS directive.

I could be mistaken, I just remember having to change that the last time I installed it for Cacti on a Debian system, but I think that was the prior version.

Try the following:

agentaddress 0.0.0.0:161

Using the address "0.0.0.0" tells most applications/libraries to bind to all addresses.