Hi,
The severity information will not be defined in the traps/notifications defined in the MIB, also severity is not defined in SNMP. Based on the trap-type(SNMP v1) or value of TrapOID(in SNMP v2 & v3) the receiving application/user can assign/assume a severity and can propagate the same along with other informations obtained from Trap PDU.
Hope this is clear.