How do I use grotesque in a sentence? (you left the noun out)
1. You're so grotesque. (Your is a personal object pronoun, not the conjunction of the words "you" and "are" as you intended)
2. That statue is grotesque. (sentences start with a capital letter).
But yes, you did use the word grotesque correctly if the person you're talking about actually is grotesque.
Usually though, if you're trying to use a word in a sentence to clearly convey the meaning of the word, you would want something more than "You're so grotesque." You could substitute the adjective grotesque with practically any other adjective. That sentence really does not help a person understand what the word grotesque means.