Today I was learning one of the…,I think it was ‘onyomi’ for the Sun radical. The hint was to think of the philosopher Nietsche (ni + chi). However, IMHO a far more effecitve hint would be for people to simply consider that nichi (+yobi) = SUN day. 
I suspect more of your learners here know the word for Sunday, already, vs having to remember ‘some guy’ named Nietsche, and then somehow connecting him to ‘sun’’
Tx!
WaniKani doesn’t use Japanese words as mnemonics for other Japanese words (even when such a mnemonic would make some degree of sense), but even aside from that, in this particular instance you’re putting the cart before the horse - you’re using 日 as a mnemonic for itself.
People here might know the word “Sunday”, but learning the word 日曜日 comes somewhat later in the process.
I definitely knew of Nietzsche before I knew of 日曜日
こんにちは is probably a safer bet if you want a well known native word.
Don’t you mean Connie Chihuahua?
I see that people can indeed know the word before the Kanji, in contrary to how WaniKani teaches.
Nonetheless, using a word for Kanji mnemonic is a risky business. People may really know little about Japanese language (supposedly?, at this level number, per WK guide).
Safer bet is well known phrases, I guess. (Though, cowはいい?)
If it works for you that’s great! But some people might not know that connection so they opt for something a bit more famous and well-known. While some people might not know Nietsche, they can easily look him up. The reading mnemonics try to replicate sounds that are already in English, especially in the beginning. As you level up there are some that incorporate previously learned kanji and vocabulary that wanikani has taught before, but in the beginning they assume that you have no background in Japanese kanji, or maybe any vocabulary.
Maybe an extra aside to say “For advanced learners, this is the reading in 日曜日!” but I wouldn’t remove Nietsche from the reading
Just out of curiosity, did you just reference ‘Kauai?’ If so, I’m not making the connection with ‘nichi.’ 
族 does…
WaniKani sometimes makes mnemonic based on Japanese words. (可愛い in this case.)
Aye, and it occurred to me shortly after posting that “Kyoto” and “Little Kyoto” are used for the きょう and きょ readings respectively, one of which just so happens to be the 京 from 京都 (though whether that counts as a Japanese word is up for argument, considering “Kyoto” is the name of the city in English too).
OMG…American, so I see Kauai. Clueless Japanese learner, so I can’t see kawai!
This works “in reverse” as well. Memorizing the meaning of 日曜日 was incredibly easy after learning 日. Whenever learning interconnected words, the first is always going to be more difficult than the second. In cases where a component goes into a compound, I’m guessing it’s easier to learn the component first.
Your case sounds like a very specific situation where a textbook was teaching vocab in romaji or kana when it should have been using kanji. I’m not sure how many of the vocabulary the learner could be expected to know prior to starting on kanji.
It’s like saying, let’s learn b, it’s the same b as in abc!
What’s abc?
We learn that later.