Yes or no. Have you ever heard of a decent player who was unable to perform because of "nerves" in real games?
I'm sure a lot of people never preform to their max potential because of nerves. On a scale of 1 to 10 , a 5 could easily play like a 2 when nervous. Nerves aren't going to make a 10 play like a 2 or anything but it can make lots of players play under their skill level. They can also go away with experience.