Is 103 lbs overweight for a 5' 2 12-year-old?

No! You're a good weight and in fact lighter than average. Statistically, you're likely to have a longer life expectancy as well. If you're really anxious about being fat, that can be a problem. According to one statistic I found, anorexia kills 0.1% of teenage girls in the U.S. Short of that, any type of anxiety is unhealthy as well as unpleasant. Enjoy your ice cream or whatever it is that you like.