Agreed, flying into any of the German airports, such as Dusseldorf, Cologne, Weeze/Niederrhein etc, even though they are not far away in a straight line, they are all a complete nightmare for getting to Maastricht by public transport, which will take you hours and involve lots of changes from any of them.
If you are coming on the train from the north, from either Eindhoven or Schiphol (which only needs one change in Utrecht unless it is very late at night - Amsterdam Centraal is also possible but a longer journey), then it is possible to change onto the local train (stoptrein) in Sittard to get to the Beek-Elsloo station which is closest and also has a bus link to the airport. I'm not sure if it really saves much time.
All the bus links and transport connections can be researched on
9292ov.nl: OV-reisinformatie en routeplanner voor bus, trein, metro, tram en veerboot The address of the airport which is the best way of finding it in the system it seems is Vliegveldweg 90.
Brussels Airport is also a good option, it is probably the easiest large airport to get to/from Maastricht. It also only needs one change on the train in Brussels-Nord (again unless it is late at night).
Many years ago KLM used to have 3 flights a day to Maastricht which was great, fly there directly, walk out of the terminal and you were in your hotel room.