I filled up the coolant a few months ago and today I found the coolant level is near MIN. Is it normal that coolant "evaporates" over time? Or does it mean that it is leaking somewhere?
No, it's not normal. If it happens over the course of a couple of months, it probably means that there is a very tiny leak somewhere and when the coolant gets hot enough, some of it escapes as steam. Check your cap's tightness first.
Or if you're unlucky and your car recently overheated, it may have caused your head gasket to blow in such a way that coolant gets sucked into the cylinder and is expelled through the exhaust, but that usually drinks up the coolant fairly quickly.