Hell no. You can use a number of languages to make a nice GUI with. It just depends on what's the usual/easiest solution for the platform. In Mac OS X's case, Objective-C and AppKit are pretty easy to use choices. However, I use REALBasic sometimes, and that allows cross platform development (and, of course, a performance hit).
So really it depends on how much work you want to put into it. You should learn Objective-C if you want to really do serious Mac development. But you can get by without it....
