Cheapness (aka “cheese’) involves performing actions that are unfair. As such, it is a touchy subject, and discussions on what is truly cheap tend to devolve. Naturally, if we called everything that could be perceived to be unfair cheap, there would be few tactics ever used, and the enjoyment of the game would be impossible.
However, there is a pretty well-understood standard for cheapness: when trying for some small set of gimmicks where there is little or no chance of escape over and over again, this is being cheap. This, at the very least, is nearly universally understood to be cheap, because it clearly requires little skill.
You should eschew the use of infinite combos and death combos. You should avoid certain types of chip K.O.’s, particularly against beginners. You should keep chicken tactics to a minimum in casual play unless it’s okay with the other player.
If there are “house rules” imposed at the location of play, these should be posted somewhere where people will notice them, or agreed on beforehand. If these kinds of rules aren’t communicated beforehand, players cannot know what is allowed!
Certain tactics may seem unfair, but in any properly-balanced game, there should be a reason why a certain tactic isn’t cheap. Obviously, intentionally using one against someone who is poorly informed will be cheap, but on someone who should know better, it is not.
Tick throws are another example; they are highly effective, and fearsome to someone who does not know what to do about them, but there are still highly practical ways to defeat them or avoid the situation altogether in well-balanced games.