Is it normal for the pipe that connects to the fan system outside to become frozen? When I first moved in 3years ago the air seemed to be very cold. I tried to keep the thermostat at 75 degrees but when weather is warm I like inside to be colder at night.

It is not normal for the pipe to be frozen. The larger of the two pipes should be very cold and maybe a little condensation on it but not frozen or frosty.
There are a number of reasons for the pipe to freeze over, here are a few of them. First a dirty filter, if the filter is dirty it can not get enough air through the filter to keep the evaporator coil from freezing. The dirty filter tends to let lint and hair and dust get through to the Evap coil and it is wet therefore every thing is going to stick to it, again slowing down the air flow. Next is the freon charge, if the charge is not right the evap coil will tend to frost over and the more frost that adheres to the pipes the less air that goes through the coil.
You could have something blocking the return air grills, such and a suitcase or a bag blocking part of the openings. It could be that you have some of the supply grills closed therefore letting less air go over the evap coil. As you can see the biggest and most prevalent problem is air flow. About 90 precent of the problem of iced over line has to do with air flow. There are more but they are something a tech should look at.