Is a yellow toilet seat normal?
Yep, sunlight will definitely change the color of plastics. It's the UV light, which is invisible to you, but also causes your sunburns.

I'd imagine you'll be cleaning this on a more or less regular basis yourself with some kind of disinfectant. I'd probably disinfect everything (a mild bleach water solution would do nicely) immediately upon moving in, just because.

If you really hate it, you could always try demanding that the landlord replace it, or you can find a replacement pretty cheap, like this one for $14 at Home Depot.

If you replace it, it'll probably turn yellow again eventually, but it'll happen slowly and you'll either have moved out by then, or will have grown to accept it as yours, or you'll just replace it again with another cheap toilet seat.

I wouldn't worry about it from a sanitary standpoint, though.  :-)