if my love for cloud, zell, and rikku is anything to go by, i had an extreme draw to blondes. mostly blondes with big personalities. as a deeply introverted person that gets exhausted from people often, i REALLY wanted to actually BE outgoing, extroverted.... and blonde.
also from my understanding, blond hair in japan is quite the opposite of what it is in the us. it's something more rebellious from what i know and associated with delinquents and otherwise social outsiders. that the al bhed are all ( i think all ) blondes i think speaks to that but then to have tidus be a blond when he's the 'outsider' of spira feels intentional character design.
meanwhile zell i wouldn't say is COMPLETELY those things - he is an avid rule follower, though he does have at least one visible tattoo and one hidden one could've pushed that more 'delinquent' apperance. also isn't lost on me seifer is blond and he absolutely a troubled child.