I haven't played too many games as Jungle with Dianna (about 20-30) but I do play a decent amount in the jungle. Personally, I don't think she's too powerful. Any early game jungler can abuse her pre-6 with constant invades, and her ganking potential pre-6 is also pretty low, flash W being your best engage. After 6, ganking is much easier but only as an all-or-nothing kind of play style. It's difficult to land your q if the enemies see you on a ward and begin backing up (comparison to lee sin, who can ward-hop and then q as gap closers, or Kayn who can walk through walls for a movespeed steroid and better angle to cut off) so if you do miss, you'll need to use R with no reset to engage. Alternatively, you can use Q after you use R to guarantee the damage for a better burst, but if your team doesn't follow up or the enemy jungle counter ganks or the enemy can kite you, you have no escape and just die. Alternatively, I would recommend Evelynn, as she still suffers pre-6 like Dianna, but is way more powerful post-6.
Overall, I think Dianna is a fun jungler, but will always be outshined in the current meta by aggressive early junglers or mid game power-spike junglers. However, when you do succeed, feels amazing. Maybe something like Predator as a rune will help secure those ganks better, give it a shot and let me know what you think.