Okay, a quick review here, since I have her filled out to 9/10 keys:
Djinn Frizz does not appear to be the strongest character, but is a lot of fun in an unpredictable way. The 33% chance of converting to permanent enrages can mean going whole matches without having it activate, or that it will happen the first time you try it. It's a pretty good amount of damage when it activates, and the random buffs are useful even when they don't get converted. Randomly being Invincible or Immune has saved my butt a few times, and getting 5 seconds of Heavy Regen can be a godsend. It's not enough to make her truly and overwhelmingly *powerful* but it's a nice little feature that I enjoy a lot and which doesn't take any extra strategy to use.
Her SA seems like it'd be somewhat annoying when she's on defense, though not enough to really make her "top-tier". The real power of Invincibility and Final Stand debuffs is to make the timer run out in longer battles, and a 5 second buff that only might appear doesn't seem likely to make much difference. Still, with new buffs coming up every 10 seconds, it'll be easy to make a mistake and fire a BB3 or some other big blockbuster just as she suddenly became Invincible, and harder to keep up long combos on her as she might pick up Thorns or Unflinching.
If you have a lot of diamonds to choose from, she might not be the best first choice. But if you have just the one, she seems like she'll serve you decently on either offense or defense, so you shouldn't be afraid to invest in her.