What's the hardest part about baking vegetables? Fitting the wheelchair into the oven.
Is this the way you see disabled people?
It's just a joke about how hard it is to fit wheelchairs into ovens.
Your insensitivity towards disabled people is a reflection of stereotypes and generalizations that are not funny. I would be happy to introduce you to some disabled people so that you can learn more about them and hopefully this would make you more sensitive to their needs.