Why the exaggerated expression still works
The reason meme swaps land is that memes are built on a strong, recognisable expression — the raised eyebrow, the side-eye, the grin. The swap replaces identity while the underlying expression and head pose come from the template, so your face ends up making a face you have never actually made. That mismatch is the joke.
It also means your source photo does not need to be expressive at all. A neutral, well-lit, front-facing photo is the best raw material; the template supplies all the drama.