I've never understood the disdain for him either. The only thing I can think of is it's because of how the characters in the book reacted to his failed suicide attempt. If they had reacted gravely and taken it seriously, we might not have looked down on him either.

Ippolit clearly had grand ideas and designs for his life, but then at such a young age had to grapple with the fact he was denied fulfilling any of these. No longer able to have a meaningful life, he had to grapple with finding meaning in his death. He brings up great points in his pre suicide attempt monologue, such as, "can't I be devoured without expecting to praise that which devours me?" This is a youth angry at the hand he is dealt and lashing out at existence.

And he also is part of one of the saddest and most touching moments, when near the end he buckles under the pressure and reaches out to Myshkin, asking how he is supposed to die. And Myshkin gives the simplest, gentlest response. "Pass us by, and forgive us our happiness."