Was Peter’s use of Joel's prophecy in Acts 2 completely fulfilled at Pentecost?
I think that it was not. I believe the verse 20 refers to a future judgment, as described in Revelation 6:12. I believe that Peter is saying that tongues is a sign, of the coming times of blessing for Israel, but which are preceded by divine judgment. That judgment began at the cross, and was poured out on Israel/Jerusalem in 70 A.D. at the destruction of Jerusalem. But that is only the tip of the iceberg. The great day of judgment is coming, and for this men must prepare by repenting and trusting in Jesus.
You may already have looked at lesson 4 in the series on Acts, where I touch on this briefly.
The Bible Knowledge Commentary says,
2:16-21. Instead of being drunk the believers were experiencing what was described in Joel 2. In Peter’s words, This is what was spoken by the Prophet Joel. This clause does not mean, “This is like that”; it means Pentecost fulfilled what Joel had described. However, the prophecies of Joel quoted in Acts 2:19-20 were not fulfilled. The implication is that the remainder would [not] be fulfilled if Israel would repent. This aspect of contingency is discussed more fully in the comments on 3:19-23.1
Here is another perspective on the issue.