Adding the Aberrant Rex to spawners via Game.ini will only work, if there are no naturally spawning Rexes (like Aberration).

On maps that have natural Rexes, the Aberrant Rexes will just spawn as regular Rexes. Unfortunately this is due to some map-issues, that only Wildcard can really fix, so if you want to have them spawn naturally on these maps, I suggest you use the custom spawner from Simple Spawners mod. You have to enter the following path for it to work:

Blueprint:'/Game/Mods/AlbireoAberrants/Rex/Rex_Character_BP_Aberrant.Rex_Character_BP_Aberrant'

This does only apply to all official maps, If the spawns work on mod maps depends really on how the creators set up the world settings for their map.