Her eyes and mouth are slightly wider than a relaxed expression, so there's visible tension. In video it could be cute, like she might just be happy, but if you freeze just that one moment then her expression is ambiguous. Either she's talking and smiling enthusiastically, or she's about to eat you enthusiastically, or more realistically she's afraid and trying to hide it. Add the creepy text and you're primed to interpret the expression negatively.