A few rare exceptions aside, if you tank a mob as a mage in a 5-man, either the tank screwed up or you did. Sorry.
No, mages aren't a high aggro class. And cloth armor makes you far too squishy to tank anything that might actually be dangerous. Heroic trash typically hits for 6k or more damage pre-mitigation, non-crit, not counting splash damage and special abilities. You may be able to survive a couple of hits, but you can forget about a healer actually being able to keep you alive for more than a few seconds.
There are, unfortunately, fairly limited contributions a mage can make outside doing direct damage the way WoW is designed.
Polymorph and Counterspell are the obvious ones (and there is some really nasty stuff in Halls of Lightning that I wish mages would help out with Counterspell more, to be honest). Frost Nova as a controlled immobilization effect is situationally helpful. Frostbite tends to hurt more than it helps, if you're specced frost. Kiting as either a frost or arcane spec can be viable situationally, but is hardly a mainstay.
To make things worse, most groups these days are too impatient to do anything that requires finesse outside a raid boss. The general mindset is that if a fight turns out to be an issue, you didn't apply enough brute force to the problem. You won't find many people that are interested in a more studied approach.