RORY: Apparently, maturity is extremely overrated in your universe.
LORELAI: That’s right. The Empress Bobo Belle forbids it.
The Empress Bobo Belle is the fictional ruler of the universe, a character that Lorelai has invented and portrays as evidence of her immaturity.
Bobo is a common name for clowns, while Belle is the French word for “beauty”, possibly to suggest “clownish beauty” for the imaginary empress, and how Lorelai may see herself.