Is it okay if baby sits in an aircon room the whole day?

I'm with the rest. I think as long as the temperature is not too cold (perhaps around 24-27 degrees), and ensure that baby is well moisturized, it should be alright. Another thing to note would be to avoid sudden changes in temperature. If you need to bring baby out, do adjust the temperature to a higher one or switch off the aircon for a short period (to let baby adjust to a warmer temperature) before bringing him/her out.