Click Damage Done and look at the Percentile number next to your name. If it's near 100, you're good. If it's near 0, you're bad. If it's over 50%, you're better than most.

The top scores (100%) are usually recorded by people with optimized party setup and strategy, highest possible gear, food, potions and rotation optimized for each encounter. If you're wondering what's a good performance score for yourself, look at your own runs in that light. What is your ilvl and gear, are your stats optimized? Did you use food and potions? Did your party skip any mechanics or push phases in a way that might increase DPS? Did your party have NIN, AST, BRD (for casters) or MCH (for melee)? How perfect was your rotation? All of those aspects are going to lower the maximum possible DPS you could have, if not optimized.