Synonyms for Skin-deep:

adj.

shallow (adjective)
surface, uncritical, epidermal, external, cursory, unsubstantial, trivial, shallow, superficial.
superficial (adjective)
shallow, cursory, trivial, one-dimensional, casual.

n.

uncritical, one-dimensional, sketchy. skin-deep (noun)
surface, shallow, superficial.

Other synonyms:

one-dimensional. uncritical. sketchy. petty
trivial.
Other relevant words:
external, sketchy, one-dimensional, uncritical, casual, trivial.