Personally, I dislike the fact Adventure tri.'s official English subs left the word untranslated. There's no good reason for it, and, with the exception of very rare cases, leaving a word untranslated like this usually leads to more problems than not.

The word "jikochuu" (自己中) used in Part 2 is an abbreviation of the word "jikochuushin" (自己中心), meaning "self-centered" or "egocentric", and is a slang word often used by younger people. It's distinguished from the word "wagamama" (わがまま), which was used for Mimi back in Adventure, in that "wagamama" is selfishness along the lines of being "spoiled" or "needy" and can occasionally have cute or endearing connotations, but "jikochuu" implies making everything about oneself and having no consideration for others, meaning its connotations are basically entirely negative. I dislike the fact it was left untranslated in the official English subs because they make it sound like it's some kind of unique or quirky term, but it's actually just normal slang and there's no need to give it this kind of special treatment. At best, it's distracting, and at worst, misleading. I think the Brazilian Portuguese subs (and potentially other language subs) definitely made a better call in translating it.

I'm guessing the English translation's rationale was that the word had to believably be one Mimi wouldn't know due to it not being "standard" language, and I think the original intended implication was that she wasn't familiar with the slang word because she was living in the United States for a time. I say "I think" because while the series has other "Mimi misusing Japanese" references, as you pointed out, "jikochuu" was already enough of a recognizable slang word to be used in a 02 drama CD from 2001 (it ranked in the top ten for the 2000 Neologism/Buzzword Award), so Mimi would actually be quite likely to know it. In my experience translating things like interviews and other official material, while translating Digimon and especially Adventure-related things usually requires drawing from older series in order to fill in proper context, Adventure tri.-related text will range from vaguely mismatching to sometimes even directly contradicting things from the original series, so the only thing you can do at such times is try to work with what you have.

Either way, I think "jikochuu" in this context could have easily been translated as something like "self-centered" or "self-absorbed"; it has a nuance of being slightly idiomatic, and it's actually what "jikochuu" literally means ("jiko" means "oneself", and "chuu" is "center" or "within", so if you want to argue that "self-centered" sounds too self-explanatory, you could say the exact same thing about "jikochuu" too). Alternatively, something like "egocentric", which is recognizable English but used somewhat uncommonly in vernacular, would probably be fine...but really, I think there's a number of possible solutions one could come up with instead of leaving untranslated and making it come off like some kind of high-level term with unfathomable nuance...

For Armor Evolution to the Unknown, I translated all of the joke Digimon names because making sure the audience knew what they meant (or could enjoy the puns) was more important than keeping the names intact. Normally I don't change Digimon names, but these were all obvious joke names, so semantics seemed more important for this case. I went with "Selfishmon" because he immediately proceeds to make everything about itself, so the nuances were obvious already. (That, and "Selfishmon" has a bit more of punch to it as a name than something like "SelfCenteredmon", don't you think?)