Let's not forget that Socialists in The States aren't really big on formal wear.I can see why it would be offputting. The meme has already reached oversaturation. Possibly the first meme to burn out in under 48 hours.
However, to try to explain why it was popular, he looked cold, uncomfortable, and isolated. Who among us can't relate to that?
As for the apparent demeanour and lack of formalwear for the occasion? He's nearly 80, it was 2'C and rather windy, comfort takes precedence over turning out well. A lot of the appeal comes from the contrast between his expression and his giant goofy mittens, which is what kids nowadays would describe as "Big Mood".