Where do you even start? He's clearly a jungler yet the AP jungle item doesn't even cater to him, so it's basically wasted gold and you're getting less value out of it compared to other AP junglers who can fully utilize it. Having poor itemization options available is a direct hit on Fiddle jungle, options should be available for all types of mages not just auto attack based ones.. Point is he's pretty much getting gimped whether he goes for runeglaive or not.
Passive is useless and underwhelming, a measly 10 mr reduction throughout the whole game? I would loove to see this passive scrapped asap. My major concern with his w, e are the insane mana costs, 120 mana for drain? It's a matter of you just getting CCed or the enemy simply walking out of range and that's it. it costs way more than it's worth.. In the jungle, very useful for sustain but outside of the jungle really not that useful at all. 130 on darkwind is also too high, not even ultimates cost more than 100 mana (although there still might be a few cases), yet a basic ability costs 130? That's pretty insane.
He has these crazy mana costs, even though counterplay is available, yet no real way to counteract the mana gating. It's not even remotely realistic for a fiddle jungle to stack a tear, but at the same time he can't even buy a proper jungle item to help him out in the first place
tldr rework fiddle