The word 'disappointing' is the present participle, present tense of the verb to disappoint. The present participle of the verb also functions as an adjective, and a gerund(noun form).
Examples:
I would describe the movie as disappointing. (noun, object of the preposition 'as')
We were notified of the disappointing results of the test. (adjective, describes the noun 'results')
I am disappointing my parents. (verb)