Its a character from Homestuck, one of the best webcomics out there. I highly recommend you check out the link and start reading, its great. The character in question doesn't appear til about a third of the way through, but she is the Best Character in Homestuck nonetheless. She's an alien, so the little caterpillar thing is her as a baby.
About Crona's poem:
Even Black*Star succumbed, and that dude thinks hes above gods so the mane six have no hope.
I haven't watched all of Soul Eater yet, so I don't really think I can say much.
Based on the video though, they'd probably be affected. I can see Fluttershy, Twilight, and Pinkie succumbing rather quickly since they're the least mentally stable of the six. RD and Rarity are more average in that area, so they likely would, too. Applejack is stable, but I don't really see her not being saddened by it.
So yeah, if any of them read it they'd be affected the same way. However, if at least one of them doesn't read it, she could break the others out of it easily enough. Twilight broke out of Discord's machinations just by reading her own letters again, and reminding the others of it released them as well. So I guess they'd recover faster via encouragement from the others, but none of them have any particular resistance to sadness on their own.
The only person I could see actually reading that poem and resisting is Usopp.