What makes an amphibian an ectoderm?

I can only assume that you mean ectotherm and not ectoderm.

Ectoderm is the outer germ layer of an embryo.... so this wouldn't make sense.

An ectotherm is a cold-blooded animal (e.g. amphibians). The animal depends on external sources to regulate its body temperature.

This is opposed to an endotherm, which is warm-blooded (e.g. mammals, birds...). They regulate their temperature themselves, creating most of their heat through metabolic processes.