。
She's a late sleeper.
Literal
She [topic-は] morning-sleeper is.
朝寝坊 literally means 'morning sleep + someone who neglects/is lazy about it' — a person who oversleeps in the morning. It can be both a noun (a late sleeper) and a する-verb (朝寝坊する = to oversleep). Used in a predicative way here: 朝寝坊だ = 'she is a late sleeper.'