There is a mental illness epidemic if you define mental illness as a mental state that is unhealthy or outside of the norm. But I am not certain their is a mental disease epidemic although I will agree certainly that there is a rise in mental disease.
What we should be asking ourselves is why there are so many mentally unwell people all of a sudden.
I think the answer is simple really. There is a rise in actual mental disease due to inorganic influences in our modern society. This has begun to come out and will continue to. But there are a lot of people who do not have mental disease whom have mental problems because of the state of the world, their life and many external stressors like social media.
For example, you have a number of people out there whom 100% believe people like AOC saying the world is over in 12 years. Think about it, if you really believed that you were in an existential crisis like that you are going to probably be stressed into a fear panic. Which is actually why people like AOC say such things.