Unfortunately not.

Map of Shibuya station

I not sure the above map is easy to understand. I have been to Shibuya station so many times over how many decades, so I can easily understand the map, but for the first time visitor...

NEx uses platform 3 & 4. There is big orange arrow connecting map of platform 3 & 4 with the rest of Shibuya station. Hope can see that platform 3 & 4 is located at southern end of platform for Yamanote Line.

When getting off NEx at Shibuya walk the platform toward north. If getting off NEx from Narita Airport then walk toward front of the train. At the northern end of platform 3 & 4 there is an escalator and a stair going up. That lead to the Central Gate of Shibuya Station, which is little bit of walk.

Once out of the Central Gate then go left till you are out side facing the road. Then make right turn toward north will take you to Hachiko exit area, which is the famous Shibuya crossing.

I think 6 hours can be more than enough time for trip to Shibuya, or may not be. Worse situation is this:

Took 30 minutes to go through passport control and customs at NRT, which can happen when there are multiple international arrivals and the line at the passport control is long. Then will be another 5 - 10 minute to get to JR Narita Airport station and buy a ticket.

What if at that moment, NEx has just departed? Then wait another 30 munities for NEx. Which mean can end up spending entire one hour just to leave the airport. Narita Airport - Shibuya is 1 hrs. 20 min. by NEx. This translates to under worse situation can take 2 hrs. 20 minutes just to get to Shibuya.

I do think you really want to be back at NRT security/departure passport control 30 minutes before the departure. Which means departing Shibuya back to Narita Airport least two hours before the flight. This will leave 1 - 1.5 hrs. at Shibuya.

If this is your first time at Shibuya or Tokyo then I will put cushion on schedule since likely you will spend some time figuring around which way to go for what.