In your base model cobalt did you change out plugs or still OEM? Should be AC Delco Iridiums in there for 100k maintenance and that's the average lifespan of them. Don't worry about 02 sensors if you don't have a CEL on. Your mpg could be from fuel leaking recall. Do you floor the accelerator pedal a lot? City based driving only? I mean by driving from stop light to stop light only and short trips would cause your mpg to decrease. Most of your fuel is consumed during startup and cold engine temps. It should take roughly 4 minutes for engine to heat up to 190-200 degrees (normal engine temp) and your 02 sensors are still in open loop trying to achieve closed loop. Your first 15 miles roughly are the worst for your vehicle on mpg, it gets better as you drive and coast/cruise.
Coby7 has some good info as well, your brakes could be slowing you down too if warped or heat spots are indicated.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille