"Cheap" is a word that has been used to complain about almost any aspect of a game. Because of this, it has no clear meaning. Some common definitions include:
One example of a strategy some players consider "cheap" is Kirbycide, specifically Swallowcide, where the Kirby player is not considered to be fighting and can produce a KO more easily than with many other techniques because some characters have few ways of approaching Kirby without the risk of getting swallowed.
As the concept of "cheapness" is nebulous and subjective, there are generally no rules for banning something in tournament play on the basis of it being cheap. If a character/strategy/move is truly strong enough to unanimously be considered bannable, it is typically regarded as being broken or significantly detracting instead of cheap. It is also entirely possible that the accusation of cheapness in unfounded, and the accuser is simply lacking critical information or execution to create a proper counterplay.
The Cheap Shot bonus in Smash 64 is awarded for overusing a single move, an action that is commonly regarded as cheap. The Cheap KO bonus in Melee is rewarded for KOing an opponent from behind, which has virtually no gameplay effect.