I have a couple, not so much a joke but funny 'thank you' cakes.
1) several gum paste sheep in various drunken states (some passed out, some slumped over a chair) with little bottles laying around. The caption would read...'Many, Many Tanked Ewes'
2) several gum paste donkeys in a row, holding hands. The caption would read...'Thank you, or as they say in Germany...Donkey Chain'
You could also do a cake that has mickey mouse and donald duck on it. The caption would read 'what mouse walks on 2 legs...mickey' The next caption would read 'what duck walks on 2 legs ...?' Most people will want to say donald but they would be wrong. The answer is ALL ducks.