This is a common pattern in English:
I don't like people joking about people with disabilities.
In this case, the phrase "people joking about people with disabilities" acts as a noun. This sentence means that you don't like the entire idea of people joking about this topic.
Here are some other examples:
I remember my mom telling me not to talk to strangers when I was a kid.
If athletes taking drugs is such a big problem, why don't the sports commissions require more frequent drug testing?