彼はみかけ倒しの人間だ。

English Translation

He is a tin god.

What does this mean?

My Japanese dictionary says:
見かけ倒し [みかけだおし] (adj-na,n) false impression, deceptive appearance, mere show

And for “tin god”:
TIN GOD | definition in the Cambridge English Dictionary

someone who behaves as if they were more important or powerful than they really are