Is it okay to mismatch alloy rims and steel rims on a car?

You're talking about a difference of maybe 6-7lbs. Not enough to worry about. If you were talking about going from a magnesium or carbon fiber wheel that weighs about 5lbs to a lead wheel that weighs 90lbs, then you would have a reason to worry, but it's not like that. Your spare is a steel wheel and a solid tire, and in some cases, are heavier than the stock wheels/tires, even though they are labled as space saver tires. Plus, many trucks do not come with a matching alloy spare, so they come with a generic steel wheel for the fullsize spare. This could also be why you see it so often, because around where I live, there are a lot of potholes from the weather (snow/ice freezing in cracks and creates potholes when the ice expands by breaking the pavement up more), which can cause a lot of flat tires.