If you want to go to Shibuya, you'll have plenty of time to do so. 90 minutes for immigration/customs is like worse than realistic worst-case scenario. More likely, it'll be 5-20min for immigration & customs combined since you're not picking up luggage. If your plane's on time, you should make the 7.44am Narita Express (even accounting for the time spent at ATM in arrival concourse), getting you into Shibuya at 9.26am. On the way back, the 14.16pm NEx out of Shibuya will get you to NRT at 15.27pm, which should be plenty good for your 5pm flight since you don't have to stop at the airline counter.

So you'll have almost 5hrs in Shibuya.