Logins are already case-sensitive, like practically everything else in Linux. If you want to use some upper-case letters in your user name, go ahead, but I think it makes it more difficult to remember the correct name.
Or do you mean creating a different user with a name that's identical except for case? Like user and User? You can do that too and the two users will be completely separate as far as Linux is concerned, but humans would constantly get them muddled up