Does epilepsy patient develop mental problem overtime?

Yeah, you need to define "mental problem" a bit better. Do they go insane? No. Will they get OCD? No. Can they get depressed? Possibly.

Depending on the situation there can be other neurological things going on; a brain tumor that causes epilepsy could cause other problems. Same with epilepsy caused by head trauma or illness. The other 70% of the time though there is no link to other mental conditions.

If it is a child who experiences them daily etc. and they are prolonged there is a chance. Most people with the proper treatments/meds are fine. There are children who are born with underlying reasons for the seizures which in that case development is a problem. Also after a traumatic injury which brings on a seizure disorder for a child or adult. When one experiences a seizure the brain is deprived of oxygen - thus possible long term effects. Some people who are taking certain meds may seem to be in a bit of a fog etc. Better management of their meds would be in order.

You are not stating WHICH kind of epilepsy we are talking here: grand mal, petit mal, temporal epilepsy etc.
In my experience I have seen no cases of patients lapsing into mental diseases but what is very common is young patients developing agressive behaviour or depressive behaviour (WHY me syndrome) sometimes leading to suicidal characteristics. A strict medication control is essential and remember: no video games or disco strobe lights.

For EMTD 65: you should share your extraordinary discovery that epileptic insults cause hypoxia with the world. I suggest the New England Journal of Neurology....for crying out loud.