I know charisma is not only appearance. Charisma also has to do with personality. Essentially, the ability to influence people with your personality.

Now, the way I see the drow culture is that it is chaotic, but, it still has a social hierarchy of sorts. Houses, matrons, female dominant etc. Most drow I envision being not only evil, but, very selfish and power hungry. There are constant power struggles happening in the drow society and it isn't always in the open. The drow are much to cunning to openly kill their adversaries to advance themselves so they engage in games of subterfuge and intrigue. They make allies that serve their purpose to advance their position. This requires a great deal of charm to accomplish. Some, obviously, would be better at it than the others.

Now as drow come to the surface, yes, they are quite untrustable. This only would require them to rely on their intelligence and cunning even more. Which in times, would require them to persuade others that they aren't as bad as the rumors say. Think of it like a used car salesman. We all know they are not to be trusted but when you are sitting in front of one, boy, can they do a good job to convince you otherwise. The fact is there are all kinds of evil beings on the surface of Thera for a drow to interact with and "use" to accomplish their means. Those other evil beings may have similar agenda's and are willing to take the risk of doing business with a drow. That doesn't eliminate the fact that they cannot be persuasive and highly charismatic.

Lastly, why would drow society not have use for a bard? I just see a drow bard being a bit darker and more macabre than other bards. Look at all the artwork in Kteng's laboratory. I always like to think of that stuff being done by a drow bard.