Mage is quite a decent and viable pick, it’s just not as god tier as a rogue or a resto druid. In general you’re not as op as you were in vanilla, but you’re good. One of the most potent casters of the expansion, just behind warlocks.
I didn’t play mage, but I played against them at rank 1 level as holy paladin, so obviously some level of bias, since they’re not as good against resto druid which is far more popular. That said, in my opinion, mages were proper broken in TBC and the only thing that held most mage players back in those days was the fact that it required some level of skill to play well (as opposed to retail nowadays, where a monkey can play one to gladiator). Not much, but players back then weren’t exactly good.
Mage Disc
Mage Lock
Both top tier comps… also 3 was pretty safe, you could also argue there are more
Mage Druid is AIDS to play against.
Mage Ele is the single most bursty comp in the entire game
Mage ShadowPriest
Mage Mage.
All of those are glad viable. Mages will always be amazing in TBC just based on the sheer amount of Arms Warriors that will be present in 2’s above 2k rating that you completely counter.