I don't know if this has ever been discussed or not, but do the characters in the Fallout world shower/bathe and brush their teeth? I'd just assume some areas with a lot of damage might not have functioned showers and baths. And by the time 200 years have passed and with no signs of production, all the toothpaste has run out? Add in the fact that I don't think I've ever seen any baths that didn't look broken and a distinct lack of toothbrushes and it looks like the Fallout characters are very dirty :P
Unless of course we're going by the fact that even in Post-apocalyptic societies, the Americans still have obscenely white and perfect teeth even when there's no toothepaste and whitener (they definitely don't go to dentists anymore!) available. Case in point, Beth from The Walking Dead ;)
I think that was just a minor oversight from the production team though, maybe the actors didn't want to not brush their teeth for weeks to give the harsh reality of a post apocalyptic world? I certainly wouldn't. Or maybe this was a failure of an example, I mean toothpaste isn't hard to find in people's houses or even shops since it's only been a year since the world ended. And we know they scavenge a lot in the Walking Dead as well. But the same cannot be said for Fallout. 200 years, toothpaste must be gone by then.