It's not unusual for shows to borrow from the actors' looks for their character models. And when you have actors like these, why not?
Archer has Judy Greer, playing the warped secretary Cheryl, (a character that borrows from Greer's previous role on Arrested Development).
Also on Archer is Aisha Tyler, playing the super-sexy spy Lana.
King of the Hill had three beautiful female actors.
Brittany Murphy, who sadly passed away in 2009, who played LuAnn
Veteran voice actor Lauren Tom (also on Futurama as Amy) plays Min and Connie.
Pamela Adlon is the voice of Bobby Hill.
Knowing what these two gorgeous actresses look like adds an interesting subtext to the episodes where Bobby and Connie develop romantic feelings for each other. Maybe that's just me.
Ironically, Meg is described on the show as being homely. Probably the animation equivalent of the TV tradition of having "ugly" characters played by attractive actors with heavy makeup.
UPDATE: to appease fellow blogger Pro Libertate, I have added the following picture of professional voice artist Kath Soucie, who has far too many credits to list here. She is most famous for her work on Captain Planet and Rugrats, in addition to doing the voice for Cubert Farnsworth on Futurama. As should be obvious from the picture to the right, she also fits in with the theme of this post.
JW pointed out that I omitted Tara Strong, who has done a lot of voice work, most notable Timmy Turner on the Fairly OddParents. In my defense, I've never seen it, or most of her other shows.