Expanding on kandyman's answer.
Origins of mukimuki: probably not German
According to the Shogakukan Kokugo Dai Jiten (KDJ) entry here, the first cited textual instance of a word mukimuki is way back in the 700s in the Man'yōshū poetry anthology. This is much older than the German word Muskel ("muscle"), of which Mucki is a derivation. Per the Herkunft ("Etymology") section in the Duden entry here, Muskel traces back to Latin musculus ("little mouse"), presumably from the way a flexing muscle looks a bit like a small animal moving around.
Origins of mukimuki: sense development
Again per the KDJ entry, mukimuki meant something more like "each going in whichever facing (muki) or direction", and this extended to a sense of "different preferences and inclinations". We see similar senses listed in the Daijisen and Daijirin entries here.
So far, none of these list any sense related to "muscular". We don't find that until we look here in the Daijisen entry for the kana-only spellizang むきむき, or here in the Japanese Wiktionary.
I can't find anything directly relevant to how the "muscular" sense arose. I suspect two possibilities:
- From the oldest sense of "each going in whichever direction", developing from the idea of "able to go whatever way", to "capable / strong" in general, and from there to "muscular".
This seems less likely, given the sizable semantic (meaning-wise) gap between "each in their own direction" to "muscular". The KDJ in particular is pretty good about listing historical development where known, and their entry for 向き向き is missing any such details.
- Instead, a vowel shift and slight semantic shift from similarly-structured adverb mukumuku ("thickly, in a billowing or bulging manner"), first cited in the KDJ to a text from 1254.
Mukumuku is limited to soft contexts, things like cotton batting or clouds or smoke. However, here we do have nearly the same word, with a similar "bulging" sense. There is at least one other example of a reduplicative adverb pair with one ending in -u and the other in -i, specifically kurukuru ↔ kurikuri, of overlapping but not identical meanings. The pair sukusuku ↔ sukisuki also have close senses and may be related, demonstrating a similar structure.
Tentative conclusion about the derivation
While I cannot find anything definitively stating when the "muscular" sense arose, I suspect it is significantly older than contact between Japanese speakers and German speakers, and indeed possibly older than the German word Mucki itself. As others have noted, this Japanese word is also much older than emoji.