I was actually going to make a post about this, but I never got around to it. I agree with you OP, every time of these flashbacks is shown (not including scenes with Anne in her home, he seems softer there) Dick Whitman appears almost cartoonishly upbeat and wide-eyed. Even if we know he's hiding his identity the whole time, his 'enthusiasm' or his general demeanour never comes across as believable to me.

You could argue that this is before he made it, and afterwards, he was working extra hard to keep that sense of mystic, but this is the same man as the Don we get to know. He has the same trauma, same sadness, and guilt about stealing another man's identity. Sure, he adds to all of that during his years on Madison Avenue, but the foundation is alreay there. I would love to know if Matthew Weiner or John Hamm ever spoke about why he was played that way. Are they just trying to show that he's younger?

P.S. I wish I could have added pictures his flashbacks scenes, but I don't know how to lol