Is it Appropriate to go to a Wedding in Jeans and a Jacket?
In a word, no. A wedding is generally a very formal event and it is important for all of the participants and guests of the wedding to be in their best attire. For men, this will generally include a suit with tie and jacket or a tuxedo. For women, an appropriate formal dress should be worn. In the warmer months, it may be acceptable for a woman to wear a sundress, but this should not be too informal, too short, or too risqué. One does not want to get noticed for all of the wrong reasons.
For underdressed people at the wedding, they will feel out of place, potentially have bad remarks made about them, and forever be shamed in any pictures that are taken. In a group of people in suits, being the only one in blue jeans will certainly stand out.
The only exception to this rule would be if the couple getting married specifically mentions casual attire for the event. In this circumstance, it would be acceptable to wear a pair of jeans and a blazer or jacket. Otherwise, being underdressed for such a formal event can be extremely embarrassing