Maybe it was different for you, but although you're right, the characters are all tragic, it doesn't read like a tragedy - i.e. it's not depressing to read. Wasn't for me, anyway.
I suspect most people, myself included, reflect on the book and remember specific qualities of the characters as humourous, because it's painful to identify closely with their actual qualities.
Oh, I seriously beg to differ. It's bleak to read. I see aspects of myself in all the characters, and I see aspects of everyone I know in all the characters. Myrna's letter about her in-pre-production movie, as read from the perspective of one surrounded by hardscrabble Hollywood dreamers, was breathtakingly heartbreaking. I'm not sure how you couldn't identify with a lot of their qualities. It's a deep-diving, expansive study on the worst aspects of hubris and vanity, leaned against each other to better concentrate their withering rays.