Jesse is surprised when Leslie wants to come to church with him. This happens in Chapter 8, titled "Easter." Here's their conversation as they're milking the cow together:
"I never knew you went to church."
"Just Easter." He concentrated on the warm udders. "I guess you think that's dumb or something."
She didn't answer for a minute. "I was thinking I'd like to go."
He stopped milking. "I don't understand you sometimes, Leslie."
"Well, I've never been to a church before. It would be a new experience for me."
He went back to work. "You'd hate it."
"Well, I'd just like to see for myself. Do you think your parents would let me go with you?"
As the conversation shows, Leslie has never been to church before, and she thinks it'd be an interesting new experience. Even though Jesse assures her she'll be really bored, Leslie isn't put off--she isn't afraid of having nothing to do or having nothing to think about, apparently. And look at how she says that she wants to see for herself whether it's boring! She doesn't just take Jess's word for it; she likes to explore things and discover them for herself.
The issue, then, reveals Leslie's curiosity, and it also shows us another way in which she and her family are different compared to Jess and his family. Because she has never been to church at all before, you can tell that her upbringing has been different from Jess's. And when Leslie willingly puts on fancier clothes and fancier manners than she normally has, you can also see how she's willing to be flexible (and even willing to be uncomfortable) in order to learn something or have a new experience.