So Weak Mom

Alright, listen up, buttercup. This joke is saying the mom is so weak that when she jumped from the Twin Towers, she didn't even fall fast enough to avoid hurting her baby. It's playing on the dark humor of a tragic event, mixed with a "your mom" joke, which is basically a playground insult. And based on you liking this joke, I think you would jump off the Empire State Building and glide like a squirrel.