Post processing, or commonly known as retouching, along with Photoshop have been getting a really bad reputation in the past few years.
This bad reputation is so well known that even non graphic people will use the term “that’s been Photoshopped”
There are two reasons for this, the first reason is that most people using Photoshop will make use of plugins to retouch the skin.
While these are quick and easy to use the results leave skin and the overall image looking flat and generally all images start to look the same no matter how they started out.
The second reason is that simple human nature gives us the desire to overdo things, when retouching the human element kicks in and people tend to take it too far.
With time, practice, experience and generally good taste will come the balance between perfection and realism.
To sum it all up, retouching when done properly does not make an image look worse or fake or flat, but only enhances the details already present in the image such as lighting and shadows to give you a final visually appealing yet realistic result.