The answer to that question is simply: he is too darn cute! I know some people find his expressions and personality annoying, but to me, they are the best thing about the series.
The truth is, I find myself making a lot of the same expressions he does, though not as extreme. And I'm a fairly shy person; I often find myself feeling awkward in public. When I first watched the anime, what drawned me to it was Mihashi, and his expressions. He reminded me of myself, and made it looks cute xD
Though Ookiku Furikabutte is not that popular in America, I hope it one day will be as popular here as it is in Japan. And I hope Mihashi will get lots and lots of fans, because he is such a cute kid.