It's not an idiomatic expression, but the structure is not "cũng + adj + được" either. "Cũng" here is an adverb that means "quite" (not "also", or "too" - it does have that meaning but not in this case). And "tạm được" is like an adjective phrase ("tạm" with the original meaning of "temporary" is used to depict the state of something that is not as good as expected but still acceptable, and "được" is an adj that means "ok, alright". So in this case it's just a normal adv+adj structure. You can also say "Cũng được" in this sentence. And you can use "cũng" with the meaning "quite" with other adjectives, such as "cũng tốt, cũng đẹp, cũng khá, cũng bình thường, cũng tàm tạm..."