I think the biggest problem there is the on resistance of your switch. I saw it was in the order of a few hundred ohms, depending on the supply voltage. That is bad. To really know what is going on, I need more information about the application you're working on.
PS: analog multiplexer is a baseband part, to switch one of many input to just one output. A mixer is a nonlinear part that translates frequencies to lower (downconverter) or higher (upconverter) frequencies. That's it!