。
She called him an idiot.
Literal
She [topic-は] him [object-を] idiot [as-と] called.
彼のこと is a softening reference — 'him, the matter of him' — that typically appears with verbs of feeling, calling, thinking, talking about. 彼を呼んだ would be more direct ('called him'); 彼のことを呼んだ adds a slight conversational warmth or framing. 馬鹿 ('idiot') here as the assigned name in the XをYと呼ぶ pattern.