Whether the mortgage of crayon Shinchan House has been paid off is not mentioned in the cartoon, but we can know that crayon Shinchan House lives in the suburbs, so it is not a particularly prosperous place. I guess Xiao Xin's mortgage should not be paid off, but it is also possible that there is no so-called mortgage at all. Xiao Xin's father (Nohara Shinsuke) is an ordinary office worker. He goes to work by bus every day and only has time to stay at home with Xiao Xin on weekends. His father is basically an ordinary person, nothing special, and he has never been very lucky in the play. His promotion failed several times, and Xiao Xin's father's income also decreased.
Xiao Xin's mother is a housewife. If she is a housewife, she certainly has no extra income. She mainly takes care of children and family, so Xiao Xin's father is the only one who really has income. Although the suburbs are not in the most prosperous urban areas, the housing prices in Japan are certainly not low. Japan has always been a country with scarce resources and a large population, so housing prices are definitely high, even in the suburbs. Besides, the house in Jia Xiaoxin is very big and expensive at first glance. Therefore, the author guesses that Jia Xiaoxin has not paid off its mortgage.
What is the current market value of his house? There is no clear hint in the play, but what is certain is that the market value should have risen a lot. Japan is a developed country, and the demand for high-grade goods such as houses is definitely in short supply, especially in the face of Japan's small territory and lack of resources, it is inevitable that house prices will rise. Therefore, Jia Xiaoxin's mortgage should not be paid off, and the market value of the house should have doubled.
All the above are my personal guesses. Please correct me if you have different opinions.