In and of itself, there is nothing wrong with wearing jeans to Mass, especially if you do not have other appropriate clothing. To the extent that it is possible, all clothing worn to church should be neat, clean, modest, and in good condition (e.g., no rips and tears). If one has clothes that are dressier, perhaps of the quality that would ordinarily be worn to an important and formal event, that clothing might be a better choice than casual clothing such as jeans. After all, one should not treat the Mass with less respect than one would treat a business meeting or a court date.
