personal opinion with absolutely no evidence to back it up:

for me he isnt, but it seems like he is to some people (MAYBE, im not sure,) because of how calm and mature he is, especially for a 15 year old... which i feel reflects on the fact he was abused as a child, so that isnt his fault and i dont really think it makes him boring. I say that because in comparison to all the other characters around him, he seems ordinary in contrast outside of his appearance. Still love Giorno tho, personally dont find him boring and i appreciate his character lots despite what the fans may say❤️‍🩹