Synonyms for Skin-deep:

adj.

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

n.

acutely, affective, basic, bitter, blind, burning, casual, deep, deep-seated, depth. one-dimensional, sketchy, uncritical. skin-deep (noun)
shallow, superficial, surface.

Other synonyms:

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