Nunu is a very supportive jungler, the point of Nunu jungle is that you annoy the enemy jungler and if able help the laners to give them an early advantage. After you have farmed enough and got a few items. You can easily do drag after a gank on bot/mid, and start teamfighting. In a teamfight wait for the enemy cc to be done cus the last thing you want is that your ult gets interupted. A good placed Nunu ult can turn a lost teamfight into a teamfight you can win.
Dont forget that.