version 1.6.9.3

Apparently the "prop" mobs such as mimics and vases extend EntityAgeable instead of MobHostileBase or MobHostileAssist. The dimensional blacklist seems to only deny spawn events for the latter two classes so the prop mobs end up spawning all the time even in blacklisted dimensions.

As a workaround https://minecraft.curseforge.com/projects/in-control can be used to prevent prop mobs from spawning as well.

[
   {
      "mob":[
         "grimoireofgaia:vase",
         "grimoireofgaia:mimic",
         "grimoireofgaia:chest"
      ],
      "dimension":2,
      "result":"deny"
   }
]

You can repeat this block in spawn.json for each dimension you'd like to blacklist.

Fixed in 1.7.0