I'm not easily offended, and the joke didn't offend me. But that being said, I didn't think it was funny, either. Generally speaking, the butt of a joke should never be a member of a marginalized group of people. Punching down is never funny, but punching up often can be. That's basically how it works. Poke fun at wealthy people, especially if they're mean, which they usually are. Make fun of bullies, especially if they're stupid and insensitive, which they always are by definition. But don't make fun of disadvantaged people, like the poor for being poor, someone with a birth defect, disabled people, etc.