Mixing would be the use of abbreviations in-game such as lol, xD, things that you wouldn't normally say in real life, or if you are trying to RP that you say the word, doing something like "Ell, oh, ell, you're so stupid" would be borderline acceptable. Words that are genuinely in the dictionary would not be considered mixing, because they can be used as common or actual vocabulary to describe someone or their actions.
someone who has just started doing something, especially playing a computer game or using a type of software, and so does not know much about it
ref.
https://dictionary.cambridge.org/us/dic ... glish/noob
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Context is very important though. The way in which you use it could be considered mixing.