I had never heard of this TV program before. As I understand it now, Morty is able to travel through different dimensions. He enters and exits the various dimensions through portals (doors; gates). Of course, in the imaginary world of this TV program, the ONLY way to travel between dimensions is by using a portal. One cannot travel through other dimensions in a car, such as a Ferrari. Therefore, Morty's answer to Jessica's question is sarcastic yet humorous. Morty's answer "Well, you know, my Ferrari's in the shop" humorously suggests that he would have come into the cafeteria in his car if his car had been running properly, and that the only reason he came through the portal, instead of using his car, was that his "Ferrari was in the shop." Then Morty immediately lets Jessica know that he is not speaking seriously, by adding "Just kidding."