That's a weird issue I can't say I've seen before. I know the Sims 3 screenshots save as JPGs instead of PNG so the quality isn't as good but it shouldn't appear as if Anti-aliasing has been turned off. Has it always been like this for you? Did you recently reinstall the game or has something changed since the last time you played?
I haven't used the game's in-game camera for a very long time. I prefer using a paid version of Fraps for screenshots because I'm a snob for quality... but still, using the game's camera shouldn't appear as you're describing. Yes, might be useful to share an example with us as Gracey suggests.