Mimicrabs have very intense spawn mechanics. If you build a base above a mineshaft, or near a zombie/skeleton spawner and leave the chests unattended, every block around will fill with mimicrabs in some time. A few will even spawn inside each other. A zombie spawner room can eventually become a mimicrab-filled room with no blocks for floor, only mimicrabs.

Additionally, they are a resource-heavy mob. After running a spark profiler in my server, I saw that around 20-25 mimicrabs caused 10% of the total server-side lag alone, which is too much considering that the server supports entity cramming mob farms with cows/sheeps without such lag spike.

I found all these in version 2.0.5 I don't know if you have already taken action in newer versions.

They actually have the same spawn mechanism as other mobs with additional condition to only spawn near a chest, and can only spawn 1 Mimicrab per chunk. You might wanna check if the config or other mods interfere with their spawn condition.

To further add on it, I've yet to see a single mimicrab spawn inside Nether Fortresses.

I can confirm they spawn in huge numbers, especially inside Strongholds.
While I do use a number of mods, among which is Optifine, none of the other mods mess with the Mimicrabs or other mob spawning mechanics.
I also use a mod called InControl to limit how many of what mobs spawn, which is a temporary fix for the massive amount of Mimicrabs.

For reference, in both a testing environment that only has Fish's Undead Rising, and a separate environment with all mods in my modpack, Mimicrabs keep spawning near chests in absurd numbers.
Again, I've only seen this happen inside strongholds.
The chests in shipwrecks and mob dungeons seem unaffected for whatever reason.
On average, roughly 20 or more Mimicrabs spawn around a chest within the Stronghold.
With one instance even racking up to 37.
This particular number only happened if there is enough space for them to spawn in.
For example, the fountain area with a chest in a space above it, inside the Stronghold.