。
She tends to look only at the surface of things.
Literal
She [topic-は] [just-ただ] things [genitive-の] surface [only-だけしか] not-look [tendency-傾向がある].
Triple-stack of restriction: ただ ('merely') + だけ ('only') + しか~ない ('only/nothing but'). All three are pulling in the same direction — narrowing the scope to 'surface' and emphasizing how limited her engagement is. うわべ (上辺, 'surface, outward appearance') is the conceptual opposite of 中身 ('contents, substance'). ~傾向がある is the formal pattern for noting a tendency or pattern in someone's behavior; it's a common piece of essay-style commentary or evaluation language. The sentence reads as a slightly critical character assessment.