Cambridge Dress Code
I think a hat would be overdoing it. Men will typically wear chinos, a collared shirt and a jacket to such things. Ladies have somewhat more choice: perhaps a dress, or smart trousers and a nice jumper. Colleges vary; some are smarter than others!

Ultimately, the most important thing is to be comfortable - the point of the reception is to meet people and make new friends, so you want to feel as relaxed as possible! So don't feel you have to dress up unduly if that's not your style.