I'm of the opinion that Azuma has quite a bit that makes her interesting. She hits much harder than any of the conventional cruisers at the tier, with her AP performance allowing her to citadel CA's at her maximum range, so it's no slouch. Her DPM on paper is low, perhaps, but especially compared to the other large cruisers, her accuracy and HE damage give her much more effective DPM due to the flexibility you get out of being able to damage targets in any situation. Her armour is lacking certainly when compared to the other large cruisers, but the key point is that her concealement (which IMO is much more important than armour) is a staggering 11.9km. You're outspotting Moskva's, Henri's, Hindenburg's, while you can bring 9 310mm guns to bear. As far as history nerds go, I picked her up primarily for this reason. She was never built, no, but the design was most certainly there as the design B-65 Super Cruiser, which was key for the night battle doctorines in the Kantai Kessen.
I believe she's a good balance between "normal" cruisers and large cruisers at her tier. Her concealement alone is enough to make her a very powerful tool when played properly and i don't for a second consider her to be mediocre.