傻乎乎 is an ABB-type reduplicated adjective that vividly describes someone as being silly, simple-minded, or naïve in a somewhat childlike or endearing way. It's more colloquial and expressive than the plain adjective 傻, and often carries a tone of affectionate teasing rather than harsh criticism. The pattern 乎乎 is a common suffix that adds a sense of 'looking or acting that way' — you'll also see 胖乎乎 (pleasantly plump) and 热乎乎 (nice and warm). Use it for people's expressions, behavior, or demeanor, but not for formal situations.