I actually prefer the artwork in Sims 3 to in Sims 2. Graphically speaking, Sims 3 is very much ahead. I didn't think they were too childish/cartoony at all, but you aren't the first person who I've heard say that.
Not to mention you can customize your sims looks so much more in Sims 3. Although maybe less "adult", it was a step in the right direction, IMO.
I am excited to see how the new sims will look though. ^_^