SNMP statistics are stored in system counters. The only way to reset the system counters is to reboot the router. You can, however, establish a baseline for SNMP statistics by setting a group of reference counters to zero.
baseline snmp
host1#baseline snmp
host1#show snmp
Contact: Joe Administrator
Location: Network Lab, Bldg 3 Floor 1
2 SNMP packets input
0 Bad SNMP version errors
0 Unknown community name
0 Illegal operation for community name supplied
0 Encoding errors
0 Number of requested variables
0 Number of altered variables
1 Get-request PDUs
1 Get-next PDUs
0 Set-request PDUs
0 Unknown security models
0 Unavailable contexts
2 SNMP packets out
0 Too big errors (Maximum packet size 1500)
1 No such name errors
0 Bad values errors
0 General errors
2 Get-response PDUs
0 SNMP trap PDUs
0 Invalid Message Report PDUs
0 Unknown PDU Handler Report PDUs
0 Unknown Context Report PDUs
0 Unsupported Security Level Report PDUs
0 Not in time Window Report PDUs
0 Unknown Username Report PDUs
0 Unknown Engine ID Report PDUs
0 Wrong Digest Report PDUs
0 Decryption Error Report PDUs