As a die-hard Fiddle main, my main issue with him in the jungle is how expensive his abilities are. Their effectiveness isn't really a problem. He currently has the choice to start Blue or never use Dark Wind - because he can't afford to use more than Drain in the jungle without Blue. Extra damage isn't going to help versus chickens unless it's enough to kill them, and it doesn't help at all versus single-target camps. (And he especially doesn't want it if he has to pay more mana.)
The problem Fiddlesticks has is when Drain is his only ability up and he still has 5 chickens to kill. The problem is that Drain costs 120 mana even if you only use it for 1-2 seconds.
If you want to make him better versus the multi-unit camps, it would be far more beneficial to have his Drain refund mana for every second he doesn't use (like it already refunds cooldown) or, better yet, refund the cost entirely if he sucks his target dry (i.e. kills the target).