Perhaps Sunfire Cape could use another buff, but I'd rather it be one more in tune with its specific goals (e.g. strong sustained defenses and damage when engaging in close combat), rather than just an extra stat to make it more attractive. CDR is specifically hard to acquire on tanks because it's meant to be a deliberate decision (e.g. "I want more uptime on my CC"), as with mages, rather than something incidental to their normal build.
Personally, I also dislike stats on items that don't contribute to their main goals, since it tends to confuse incentives when purchasing them, and in the long term can be detrimental to itemization as a whole because items end up being picked based on the heft of their stat list, rather than their niche (that's already somewhat the case now on some items, particularly tanky choices like Righteous Glory/Dead Man's Plate/Sunfire Cape). Perhaps CDR would be more relevant on Sunfire than on most other tanky options, considering how the item's all about sustained effectiveness, but it still doesn't really directly enhance the item's function, whereas health and armor let the user last longer when sticking around to apply the burn aura.
Seeing how Sunfire got buffed only recently (i.e. last patch), it might be worth waiting a little longer to see if it really needs another top-up to reach the level of popularity it deserves, but if it's still not there yet it might be worth jacking up its unique strengths even further. Taking its most recent buff, dropping its bonus damage against monsters and instead having the burn deal 100% bonus damage to minions would reinforce its niche as a waveclear option for tanky top laners, for example, and adding back Cinderhulk's old ramping damage could emphasize its function as a source of sustained dueling power, as opposed to DMP's focus on initiation-centric damage and CC.