By Definition, Netcode refers to the application code that handles how data and instructions are passed from computer to computer during a multiplayer game, whether it's across a LAN or modem.
What it is not
What it is not is...
a poor connection, a poor connection is a result of poor internet.
Being dropped from a game, If it happened all the time, to a multitude of people then possibly it could be the net code. However in most games (Mortal Kombat) dropping is not the result of poor net code.
changing, One thing to remember about netcode, is it is code. Code is the same every time, therefor if one match is laggy and another is not, the netcode did not "get worse"