Instant coffee is actually the same coffee that you brew from coffee grounds but instead instant coffee is made to be put directly into hot water and stirred up to make an instant cup of coffee.
Instant coffee is no worse for your kidneys health than regular coffee brewed from coffee grounds.
The instant coffee may actually be healthier for your kidneys and health than brewed coffee from coffee grounds because it contains less caffeine.
Instant coffee contains the same antioxidants though as regular coffee but regular coffee tastes much better than any instant coffee does.
If you only want to quickly make a cup of coffee I would recommend getting a Keurig coffee maker that makes one cup of coffee at a time and it tastes much better as well.