No, not talking about the jelly donut misconception (though if you want to comment on it for additional context, go for it.) I mean, why did call himself a Berliner? What is the historical context? If it was about solidarity, it would make sense to say general things about supporting Berlin during the Cold War, but instead, he effectively said he was a citizen of Berlin. Did he have a personal connection to the city? Do we know who came up with the idea of him saying this? Did he or his admin know how well the people would respond or understand why he said?
As for the why he said it, to interpret the speech, look to the prior phrase where he quotes “civis Romanus sum,” which is a statement Roman citizens could use to establish their rights and guarantee safety while traveling the empire.
The complete quote, which doesn’t appear in note cards for the final speech, was:
“Two thousand years ago the proudest boast was ‘civis Romanus sum.’ Today, in the world of freedom, the proudest boast is ‘I am a Berliner.’”
He was declaring solidity with the Berliners.
However, the “jelly donut” issue is a misconception. He practiced that phrase until moments before the speech. The Council on Foreign Relations explains why it is an urban legend he called himself a jelly donut.
Source : Council on Foreign Relations (their post on this is probably the most concise explanation. Others are much more lengthy.