It's because he never smiles with his eyes, at all.
I don't watch his content and I'm not familiar with him whatsoever, but every photo I've seen of him with that smile just looks like it's a fake, forced smile. Maybe that's actually a genuine smile and his face just does that, I don't know. But yeah, it's definitely off-putting to me, as well.
I think he means that when most people genuinely smile, the muscles around their eyes also pull up. However it looks like Mr. Beast is just pulling his lips into a smile and not his full face. This makes it feel like the smile isn't genuine.