ugly? hell no, a lopsided grin just adds +10 charm.
i don't know what my case is, but one half my face is way more expressive than the other. i know i've also had times where i felt self conscious about it, but after realizing people with even more extreme asymmetries have the nicest smiles i've ever seen, i've decided to take it for what it is. nobody's perfectly symmetrical anyway, so most folks won't notice at first glance--and if they do, that doesn't make it a bad thing! you just gotta let that natural light shine through and roll with it.