A couple of thoughts:
First, strictly speaking, this isn’t a footnote to the Gospel, per se – it’s a footnote to a particular edition of the Bible. I know it sounds like I’m nit-picking, but it’s important to recognize that a particular footnote belongs to an edition – not all Bibles will have that footnote there.
Second, though, I think I would emphasize that the notion of ‘ambassador’ isn’t what’s primary here. After all, the Scriptural text doesn’t use the word ‘ambassador’ at all – it says “he who, having sent me, is with me.” The word ‘ambassador’ shows up only in the NAB’s heading, as a way to express that Jesus is sent by the Father.
Finally, I would say that the important thing that this footnote is pointing out is that Jesus is from God; His knowledge of God doesn’t come second-hand, but comes from His intimate relationship with God. Therefore, it’s not about being an ‘ambassador’, per se; that is, it’s not about Jesus’ role, but about about His identity.
Now… to your question: strictly speaking, a prophet brings a message from God to the audience whom God wishes to reach; therefore, a prophet is an ambassador.
