Sprinkle gelatin over 1 cup cold water in a small glass bowl and let stand 1 minute. Microwave on high until hot and gelatin is melted, about 1 minute. Add sugar, whisking until dissolved. Whisk mixture into watermelon juice. Divide mixture among 6 glass dishes (about 8 oz. each). Chill until set, about 2 hours.
Top each serving with a few melon balls, a dollop of whipped cream, and a sprinkle of mint.