OK, here’s the deal. “We as writers” need to (mostly) use words our audience can understand without stopping to look them up. That’s the vocabulary imperative for any writer. “Fleek” is not going to make the cut long term or short term, no matter how “fleek” it is. Another important point for us, as writers, is that no words are inherently “bad” or “good.” There are words that are the right words for the job and words that are not the right words for the job. The badness or goodness of a word is not inherent in the word at all, but in its fitness for use.
Lots of words pass through our world as fads. Some of them are really groovy, but they exist for a short-lived purpose. Sometimes words are employed by the young to separate them from the larger tribe. I get that. I used to be young, and so much of what the older people did made no sense. I now see that youth’s special vocabulary came from a deep (atavistic?) desire to reciprocate the incomprehensibility. Golding wrote about this in Lord of the Flies.
Words are the paints of writing. The reason we make kids learn “big words” is so they have a bigger paintbox. Who knows what they’re going to be when they grow up and stop being groovy, phat, bad, wack and fleek? The right words give personality, place and time to characters in a story, precision to a technical document, clarity to an analysis of an argument. The right word can circumscribe a sunset, hold a mountain range, light up a river. The wrong word strikes a discordant note so abrasive that readers flee.