Yes, it's perfectly OK to use mods for the Witcher games. We wouldn't have subforums dedicated to modding on the forum if it wasn't. 
Only thing you need to keep in mind that mods are 3rd party additions and therefore not officially supported. So use them at your own risk (Nexus comments are good for finding out about potential issues the mod(s) may cause).
Only thing you need to keep in mind that mods are 3rd party additions and therefore not officially supported. So use them at your own risk (Nexus comments are good for finding out about potential issues the mod(s) may cause).