I am beginning to understand the difference between ser/estar and in what contexts they are used. Ser is a priori a permanent feature and estar the temporary state of being. I am wondering if it is common to use them interchangeably in a humorous way. Ex. *I brag to a group of friends and say, "Yo soy rico, fuerte, y guapo." and then someone responds, "ESTAS rico, fuerte, y guapo." The gist of the joke being that the good thing will be taken away somehow.
That's actually an interesting question. I can't exactly answer it, but I can tell you that using one or the other can drastically alter the meaning of your sentence. For example: Estás loco (You are crazy [in a joking manner]) Eres loco (You are crazy [you have a mental problem])
The way you phrased the joke, you're implying the person may lose his riches, strength, and good looks. The joke wouldn't really work that way. Basically, one of the verbs should be used based on the adjective you're using. Switching the verb usually imply the state is temporary, it's just the wrong word. Even native speakers get it wrong occasionally, but they're usually understood.
Sometimes you can make a pun when the meaning of the adjective changes, like in the estas/eres loco example jack_fosho gave. The difference is the meaning of the word changes instead of implying the state is only temporary.
For example, there used to be a billboard near my house that showed a woman holding a beer with the message, "Esta buena y es buena." In this context, what they said was the beer was good and the woman was sexy. But, they said it in a way that came off as clever word play in Spanish.
Your joke would definitely work, albeit more as a cute comeback than a real joke. The only problem, which doesn't defeat the joke, is that "estás rico" would usually be interpreted as "you're hot" (less often as "you're rich"). But if the shoe fits, the phrase would definitely work. It would even work as sarcasm if the guy is not actually hot... [Note: physical attributes often require the verb "estar," but you can use "ser" if you want to emphasize that those qualities are durable. E.g.; "Estás guapo" vs. "Eres guapo"].
"Ser is a priori a permanent feature and estar the temporary state of being."
This is also usually how it is explained in many Spanish courses, but I would like to warn that there are many exceptions to this - almost too many for it to be a useful rule at all. The most obvious example is perhaps "estar muerto", which is how you say that something/someone is dead. Being dead is quite permanent, but doesn't use "ser" - and there are many, many examples like this.
(el vaso está roto, son las nueve, el concierto es en el estadio, etc etc)