Yes, fake sunglasses are bad for your eyes since they are harm to your vision and your eyes.
Some people are glad to wear fake sunglasses because they think fake sunglasses look fashion and are very cheap. It's right that sunglasses are very cheap, but they have poor quality when compared with real ones,because of their poor styles ,materials and workmanship.
Besides, the lenses may damage your eye since they don't provide adequate UV protection or they may have none at all. This puts you at risk for other serious eye problems caused by harsh UV light. Furthermore, the lenses may be distorted, giving you headaches and visual disturbances.
so, I suggest you pay more attention to the quality of the product instead of its prices. Choose a real one for yourself.