I know this is an old post, and I'm not Japanese, but I did some research and I found out this:
Japanese don't look at the mouth to notice emotions, rather the eyes. Since the "eyes" here are literally just stripes, I doubt they'd see anything.
Example of Japanese emoji: (^_^)
Example of English emoji: :)
These represent the same feeling (happy), but there's a stark difference, as you can see.
This is because the Japanese learn to read eyes more than mouths, because in Japanese culture, people tend to suppress emotion, but eyes are harder to control than mouths, so they're often easier to read in that case.
"when Yuki entered graduate school and began communicating with American scholars over e-mail, he was often confused by their use of emoticons such as smiley faces :) and sad faces, or :(." - https://www.livescience.com/1498-americans-japanese-read-faces-differently.html (Thats a source, btw)