The comedy is created by subversion of your expectation that college degreed people would not be working at a fast food place. The interaction is meant to be read initially as a neutral status interaction and then slides into a upper to lower status interaction as the post reveals that the answer to the implied question from the customer is that the cashier has an art degree. The initial humor is at the expense of the cashier. The next part of the joke reveals that the customer is, in fact, of true lower status of the two because they don't understand the horror of a world that will result from devaluing those with art knowledge, exemplified in the joke as those with art degrees. The art degree here is a stand in for our capacity for human empathy and connection. What fools we would be without it. What greater fools could we become if we actively refused to cultivate it. We could become evil, and that fact, that true evil that can exist and we could have blindness to it or even become it, is the comedy here. The banality of the customer here, the interaction, the shittiness of it all, that is the comedy. How this helps, it was not generated in any way by AI and it's fuckin sad that I have to say that.

it suggests that art majors are not employable in their field of study and thus end up working in fast food

It suggests that art and literature are worthless, and if you have such a degree you’ll be working fast food, known as among the worst jobs (low paid and nobody has a lot of respect for fast food workers).

That all the arts degree will be good for is working in food service