Twilight is the princess of friendship, and that means she has all kinds of responsibilities ranging from friendship, to starring in a cartoon, but only in a meta sense because she doesn't star in a cartoon within her universe. She IS a cartoon. And a horse. And even though she's such a busy cartoon, her wonderful friend Rarity comes to ask Twilight for valuable friendship help with a loan.
In spite of having a princess and a giant crystal castle that overshadows the town, lowering property values and making it harder to grow things, Ponyville still enjoys the perks of a democratically elected mayor! She does a lot of important things, such as delegating her responsibilities to Twilight Sparkle, because Twilight is able to make lists and spreadsheets.