That の is a nominalizer and が is omitted after it.

> ［［客が分かっているの］（が）大前提］の　ツンデレカフェ

分かる is punctual, meaning it refers to an instantaneous change from a state of not understanding to one of understanding. If you are already in the latter state, you should use 分かっている. (Calling this whole thing the て-form is misleading because the て-form of 分かる is only 分かって.)

客が分かるの（が）大前提 would mean the cafe assumes the customer *will* understand the rudeness of the maids (when they are treated with it). 客が分かっているの（が）大前提 means the cafe assumes the customers already understand they will be treated rudely.

Etymologically speaking, 分かる is the intransitive, and “spontaneous” version of 分ける (“to divide”). Something blurry in your head *gets divided* and boundaries become clear. In modern Japanese, 分かれる makes up an intransitive-transitive pair with 分ける. This is also punctual (“to divide itself” or “to get divided”), not stative (“to be divided”).