Life is a little hard for Mexican restaurants in Shanghai. Mostly it is because Mexican Cuisine is not favored by Chinese. An example is Taco Bell. They came into Shanghai in 2003 but they had retreated in 2008 due to not having many patrons. Overall there are not many options for Mexican restaurants in Shanghai. Following is a list of Mexican restaurants where you can have some Mexican flavors although you can not expect them to be very much "authentic" Mexican cuisine.
All About Shanghai www.allaboutsh.com

Mexico Lindo (墨西哥灵得餐厅)

   

Map! Find where it is!
Address: No.39 Hong Mei Leisure Street, Lane 3338 Hong Mei Road, close to Hong Qiao Road (虹梅路3338弄虹梅路休闲街39号,近虹桥路)
How to get to: Taxi.
Tel: 021-62622797.
All About Shanghai www.allaboutsh.com

Zapata's Mexican Cantina

Map! Find where it is!
Address: No.5 Heng Shan Road, close to Dong Ping Road (衡山路5号,近东平路)
How to get to: Heng Shan Road Station(衡山路站) of Metro No.1.
Tel: 021-64746166, 64334104.
All About Shanghai www.allaboutsh.com

Taco Popo

Map! Find where it is!
Address: 3F No.78-80 Tong Ren Road, close to Middle Yan An Road (铜仁路78-80号3楼,近延安中路)
How to get to: West Nan Jing Road Station(南京西路站) of Metro No.2.
Tel: 021-62893602.