"My grandfather is funny."
Translation:Mi abuelo es divertido.
If you're thinking of the "personal a," it's only used before direct objects. Here "my grandfather" is the subject.
I admire my grandfather. (Yo) Admiro a mi abuelo. (direct object)
You'll also see "a mi abuelo" used in sentences where it clarifies the indirect object: A mi abuelo le gusta correr. My grandfather likes to run. (Running is pleasing to my grandfather.)
I wondered the same thing, but I just learned that although it may be a temporary state, the estar verb form is not used here. The ser form of verb is also used for physical descriptions, character descriptions and personality traits. In this sentence, es is correct because it explains his personality trait.