Lorelai is just so very clearly resentful of April because she feels that it threw off her wedding plans. And yes, it sort of did, but she needs to be more supportive of Luke and getting to know April. She needs to be there for Luke because then maybe he would feel more secure/settled with the situation and be ready to get married. She also needs to express her concerns to Luke and let him know that they need to have a date set within the year