Desperado isn't too bad -
http://www.desperado.ch/ - just really expensive for what you get, and they add odd things in there (I'm used to Tex-Mex though).
The best thing is to go to El Maiz on Josefstrasse behind Zurich HB and buy your own ingredients. They're a little pricey as well but it won't cost near as much as going to one of the restaurants and being disappointed. You can also order online.
http://elmaiz.ch/oscommerce/index.php?cPath=37
Stay away from the Mexican Restaurant in the Niederdorf - unless they've changed drastically over the past few years you'll be very disappointed
I make Tex-Mex at home all the time so if you need any help with recipes/where to get things just let me know.