Erdem, the short answer is no. Sebum is not in itself harmful- it's just a way the body tries to naturally moisturise itself. However, bacteria and fungi tend to proliferate in the sebum, especially when there is too much of it. This can cause problems.
Your doctor thinks you have sebhorric dermatitis on the lips.
The cause of
Sebhorric dermatitis is not totally clear but we know it occurs when the skin produces too much sebum and fungi proliferate in the sebum, causing the body to respond with an inflammatory reaction (redness, crusting etc). Hyperkeratosis is part of the abnormal skin changes seen in sebhorric dermatitis.
I initially said I thought that Exfoliative Chelitis is not just sebhorric dermatitis on the lips because sebhorric dermatitis effects areas of the skin where there are a lot of sebaceous glands and the lips have no sebaceous glands (and thus no sebum ). However, it is possible to have unusually placed sebaceous glands on the lips (fordyce spots), thus making it theoretically possible to have sebhorric dermatitis on the lips.
It's not clear though that EC is equivalent to sebhorric dermatitis . My informed guess is that they are not exactly the same thing but I could be wrong about that.