Mei Ling is not plot relevant now, perhaps. However, it's POSSIBLE that she's been foreshadowed to be plot relevant (her Touhou Hisoutenko ending). In that regards, her secretly being a dragon but ZUN finally revealing it when she's moved to the forefront is a possibility
(note: It's a possibility, not a PROBability. Big difference, but in this case, this entire thread is just about possibilities so that should be enough)
Also, dragons are youkai. They're in Perfect Memento's Encyclopedia, after all. The whole point of Perfect Memento is to document youkai...
In regards to intelligence, Mei Ling for the most part seems to be portrayed as "Mediocre" or "average" (perhaps slightly below average). She's easily fooled and often accidentally makes mistakes (Inaba of Moon and Earth, Strange and Bright Nature Deity, etc), but she doesn't go around making ridiculous leaps of logic like Cirno and Utsuho seem to do (her conversation with Marisa in EoSD is more because MARISA is the insane one, which would confuse anyone, really). She's probably meant to represent your average Joe (Jane?) in most respects. Of course, considering that the rest of the Scarlet Devil Mansion are geniuses in most regards when you really study them, this does make her look bad, but only relatively.
Her mistaking that giant robot in Hisoutenko to be a god of destruction was in part on purpose (Her profile says she might have exaggerated it just a little cause she wanted some excitement)
Of course, it still doesn't seem to be in the league that you'd expect a dragon to be when it comes to brains. Admittingly, I'm not sure if it was ever stated that dragons are smart, but still... well, that's why my own theory is that Mei Ling's been hit with a serious case of amnesia or so. Or maybe she's just immature (IE, a baby dragon, metaphorically speaking)