i feel like a big reason for why nico is known as the character with the most trauma or the most tragic character- regardless of if he actually is or not- is simply because he had a very clear and strong reaction to it
for other characters that have gone through a lot of trauamtic events we never see too much of a reaction aside from when it just happens. we hardly see percy even think about the bullying or abuse he went through, annabeths pov in hoo gives us almost nothing about luke or her father, leo is more well known as a tragic character but even for him he ignores his trauma with humor
though another big reason for why nico is seen as so tragic is because of how old he is, but by the time hoo comes around rick has forgotten his age so much that he made him basically the same age as a few of the seven and of course his age also is a reason for his such strong reaction. for a lot of these characters, we dont see their most extreme traumatic event happen in real time and mostly only hear of it, but since we see nicos direct reaction to the traumatic event, and considering his age and how dependent he was on bianca, we can see how it affected him a lot better than we can for other characters
basically nicos trauma is a lot more of his personality than other characters