Can I format alert message from a SNMP trap?

What you want is a dynamic template processor, but all we have is simple token substitution and no indication that will ever change (I have asked repeatedly for years).  You can route alerts to an external integration, which is how we handle transformation of tokens, but you lose some stuff when you do that depending on how you integrate.  For example, we use external email integration into our ticket system with a filter that handles the transformation, but custom email integrations do not get the same handling as builtin email for certain things (e.g., you do not get ACK or SDT notices).