。
Hunger is bad for your mental state.
Literal
Hunger [topic-は] mentally bad is-[copula-だ].
メンタル的に ('mentally') combines the loanword メンタル with the productive ~的 adjectival suffix + に for an adverbial. A modern casual phrasing that would have been 精神的に in older/more formal Japanese. The sentence is a simple health observation.