Please be advised that SysEDGE 5.1 is no longer supported (and hasn't been since August 2014):
CA Virtual Assurance for Infrastructure Managers and CA Systems Performance for Infrastructure Managers 12.0 SP02 (12.0.…
Please also note that only SysEDGE 5.8.2 and 5.9 are currently supported:
CA Virtual Assurance for IM (VAIM) Releases 12.6, 12.7.0, 12.7.1, 12.8.0, 12.8.1 End of Service Announcement - CA Techno…
The oldest MIB that I have immediate access to is 5.6 (which is just one version newer from 5.1 despite the larger version jump) and 5.6 should most certainly support severity. The severity flag and aggregate monitoring was one of the big features in SysEDGE 5.x (from the initial 5.0 release) and most certainly should be supported in 5.1 as well. I suspect that there would be something blocking this on the Spectrum side (either due to the version of Spectrum or Spectrum displaying different options depending on the version of the agent available).
I would recommend updating the agent regardless.