Now that Bart and family have made it into the TV mainstream and gotten their mugs on just about every T-shirt in the country, the Simpsons juggernaut thunders on with a new magazine, Simpsons Illustrated. The quarterly (cover price: $2.50) will ''explore the underbelly of the Simpsons, looking at the wonderful world of Springfield, and taking fans places they couldn't go in a TV show,'' says Katy Dobbs, SI's editorial director. That means an up-close- and-personal look at the family fridge, Marge's shopping list (including eight variations on pork), the news that Bart's original hairdo contained 13 spikes (there are nine now), and a three-page pin-up of the more than 50 characters who have appeared on the show. Capping the first issue is ''Bart's Bottom 40,'' a list of such pet peeves as: ''the hardened crust on the top of mom's casseroles,'' ''cartoons with redeeming social messages,'' and ''Lisa's saxophone.'' What's next, a swimsuit issue?