I tried to create an ender dragon mob dimlet, but the dimlet is not craftable. I tried the given rarity 5 and also 6, none of them worked.

Using version atm 3 v5.3, playing on my server.

we've known about this for a while and I don't think we (ATM team) changed it intentionally -- we've assumed it was a change in defaults from RFTools dimensions.

Thinking about it now though, I might add a custom recipe for this outside of rftools dimlet workbench anyway.

we disabled the syringe on the dragon since it could be abuse in the rft spawner for griefing in other dimensions by spawning a dragon there

starting in 5.7, there will be a crafting recipe to craft a dragon dimlet outright to be used in RFT dimensions

Uh, you know you can disable spawning ender dragons in the spawner without messing with the syringe, right? The way you did it is unnecessarily confusing a lot of players: McJtyMods/RFToolsDimensions#133

The ender dragon dimlet specifically states it is not craftable in the 5.8 version in JEI. I think that is an RF Tools thing not an ATM custom configuration. The implications is that you may only be able to find it by searching dim loot rooms.

I just checked in a clean instance with just RFTools (the same version). It's not an RFTools thing.

Well then based upon the comment it is a specific pack configuration and is intentional.

My point is that you can accomplish the stated goal (keeping people from using the Spawner to spawn ender dragons) without breaking other uses of the syringe.

Actually, it's not just ender dragons. ALL mob dimlets are broken. I'll open a new issue about this.