Currently Control and Poke mages are fairly strong, the sustained damage mages like Cassio are in a weak spot, and I'm not sure about burst mages, as the closest thing I play to a burst mage these days is Xerath, Orianna, and Kassadin, who are poke, control, and assassin respectively.
Cassio in particular is in a weak spot as she needs to scale up and games are so fast right now.
Riot will very likely nerf Frost Queen's Claim, they took an already solid item, and added the active from twin shadows, which was sometimes built just because the active was so good despite the item itself being pretty awful itself.
Overall I don't think mages are super weak, they're not in a great spot, but they're still viable if for nothing else than you need magic damage on your team to prevent a tank from taking over the game. It's certainly much more difficult to carry with mages at the moment though. I think because it's more difficult to carry with mages is the exact reason it's more valuable to take champions that offer more than just damage. Orianna offers solid shields, speed buffs, a game changing ultimate. Xerath has nearly unparalleled wave clear, the ability to apply his damage from safety, and major damage. Lulu has so much utility it's insane and she negates any assassin. The list goes on with similar things and similar champions.
Mages who can siege well are extremely valuable, since towers are so squishy, wave clear and poke to get people away from your tower is very effective. Cassio does okay clearing waves, but is not very good a sieging due to her lower range abilities.