Well, as someone who unashamedly sucks at the game at any remotely competitive level, I think that casuals and scrubs are 2 very different things. Being a scrub isn't a skill level. It's a mindset.
Using the spamming argument as an example: On the one hand, someone who accuses someone who beat them through plain and simple good fighting of "spamming" is a scrub- they are blaming their own defeat on the other player "cheating" even if they won with diversified and well thought out moves.
However, it is a well known fact that some characters have easily spammable attacks, often ones with exceptionally quick animation times and poorly done hitboxes, which whilst easily beaten by an experienced player (so not me) are very effective against less proficient players, such as new players and casuals who simply put, aren't skilled enough to know how to deal with these techniques. Natsu's groundbombs are an example. Using them in friendly/casual games or against less skilled players is fairly poor form and kind of a dick move.
A more skilled (but not necessarily very well skilled) player deliberately seeking out casual players who just want to have a good time or new players who don't know a nodachi from a zweihander and then using these easily exploited techniques on them, before proceeding to accuse them of being scrubs and telling them they need to "git gud" are themselves scrubs- these players are often themselves not very skilled beyond the easily spammable moves, and for all their crowing about being "pro" and "owning you scrubs" would be utterly demolished by a player that actually knew what they were doing.
I know a player who is superb at Natsu- he doesn't use the same combos over and over, and can wipe the floor with me regardless what style I use. However, he isn't a dick about it; he freely admits that he has an unfair advantage over me with regards to skill and doesn't always pick Natsu against me (although he always beats me anyway). Furthermore he is actually skilled at the game, being in the top ranks and beating basically everyone he fights.
TLDR: Scrub = dickwad + basic understanding of exploitable techniques + ego - actual skill.