You are bad if:
Assuming everybody in the raid group played at your skill level, the group would be not be capable of downing the current difficulty of raid you are in.
So bad for heroic may be more than fine for flex. DPS varies of course my gear, but for LFR it is fine to assume 496 (for siege). Of course if you are at 540 ilvl and only doing like 110k, you would probably be around 60k at 496 so you would be bad.