In TOS, at least, they didn't all have bowl cuts, Sarek and at least one of his aides in Journey to Babel had somewhat different haircuts as did Stonn in Amok Time and Mirror-Spock's Vulcan bodyguard in Mirror Mirror. We might not have seen all that much in the way of variations in those episodes, but it was more than we saw in all of TNG era Trek.
Come to think of it, we saw even more variation in TMP with the one male Vulcan Master with hair, he definitely does not have a bowl cut.
Come to think of it, we saw even more variation in TMP with the one male Vulcan Master with hair, he definitely does not have a bowl cut.