It's worth noting that the mouseover text isn't really accurate, or at the very least it's misleading. It's true that lexical words in English have to consist of at least one heavy syllable, meaning that they need either a short vowel followed by a consonant or a diphthong or long vowel, but that constraint doesn't apply to function words and interjections. The articles
the and
a both end in a lax vowel, as does the reduced pronoun
ya, and so do many interjections, including
huh and
yeah. So
meh is not phonologically unusual at all.