No, you don聮t want a mountain bike for around-town riding. The thing probably weighs as much as you do. Fortunately there are lots of nice, fast hybrids out there.
Bianchi Boardwalk Bike

Marin聮s Novato (marinbikes.com), for instance, has a light aluminum frame, fast tires mounted on 26-inch wheels, and smooth-shifting Shimano running gear. And it聮s only $529. It聮s available in a 13-inch frame size, which should be about right for you.
Scott聮s Sub 30 Classic (scottusa.com) offers a very similar frame/running gear combination, for just a few bucks over $500. Its 700C wheels might offer a little less rolling resistance, although they might also be a little too tall for you. The Sub 30 comes in a 聯small” frame size, plus larger ones.
Lastly, there聮s Bianchi聮s spiffy Boardwalk ($500; bianchiusa.com). It opts for SRAM shifters, an increasingly popular and very functional choice. And while its frame is aluminum, the front fork is steel. That results in a smoother ride than the aluminum forks on the Marin and Scott bikes, although it also adds a touch of weight. Plus, I love to say 聯Bianchi.” Try it! Bee-awn-kee. Then say, 聯Un bicicletta e molto favoloso!” Isn聮t that fun?
Anyway, try to find a shop that has at least two of these bikes (or something similar聴in this price range all hybrid bikes are very similar) and give them a spin. If you really intend to mostly ride around town, then ask the shop to swap out the tires for something a little skinnier. The Scott and Marin bikes both come with relatively chubby 700X32 tires. 700X28 would be better.
Ciao!
Check out 国产吃瓜黑料聮s picks for the . They all share one basic feature: fun!