LUKE: Oh, you have a kid, so you know everything, right?
LORELAI: I have a kid, so yeah, I know a little more than you do.
LUKE: You know, you ever think maybe you just got lucky with Rory? I mean, you did get pregnant at sixteen. That doesn’t show the greatest decision making skills, now does it?
Luke is furious to discover that Lorelai has taken it upon herself to start lecturing Jess without him being present, and thus doesn’t take on board the serious information that Lorelai is giving him – that Jess stole beer from her fridge. Maybe Luke should have listened to her, but on the other hand, Lorelai is too angry and upset to handle it very tactfully.
I think Lorelai is correct that Luke could avail himself of what parenting advice she has to offer – she has raised a fairly well-behaved, academically successful child, after all. She must know something about children and teenagers. (And she got pregnant at fifteen, not sixteen).
However, I think Luke has a point that she has got a bit lucky with Rory, who’s a fairly easygoing, compliant child (she takes after Christopher and his mother in that regard, rather than Lorelai). If Lorelai had had a kid more like herself, rebellious and oppositional, particularly a boy, she might have found being a young single mother rather more challenging.
This is the beginning of Jess putting a strain on the relationship between Luke and Lorelai, and serves as yet another obstacle to them getting together (Jess sharing a one-bedroom apartment with Luke would surely make that a bit problematic, even if there were no other issues).