Do you like regular exeggutor or alolan exeggutor? Also, why do you think alolan exeggutor is part dragon but the other is psychic?
Well, the Pokédex says “as it grew taller and taller, it outgrew its reliance on psychic powers, while within it awakened the power of the sleeping dragon,” which is clearly meant to sound like it’s explained something without actually telling us anything. Alolan Exeggutor doesn’t learn any traditional Dragon-type moves, and there’s nothing especially dragon-ish about his Dragon Hammer attack. I always used to think that Dragon Pokémon were defined by their connection to the Pokémon world’s abstract “life force,” but that doesn’t really help much either (except insofar as it suggests that Grass/Dragon should probably be a more common combination than it is).
BUT the good news is, there is a kind of palm tree called the Dragon Tree, Dracaena marginata, which is native to Madagascar and not Hawai’i, but is at least suitably tropical. So… I guess maybe that counts?