The English spelling is the fossil. Mong Kok preserves a name that the Chinese stopped using ninety years ago.
Sometime in the 1930s the written name changed. 芒 mong4, the weed, became 旺 wong6, which means flourishing, busy, prosperous — a word you’d want over a shopfront.
The two words don’t sound the same in Cantonese — mong4 and wong6 differ in the initial and the tone — so this wasn’t a spelling drift. Somebody chose it. What we don’t have is a document saying who, or when exactly, or whether anyone objected. The county gazetteer records the old name; the modern signage records the new one; the change itself happened in the gap.
The English is now the archive
By the time the place was being written in Roman letters in any quantity, the sound being transcribed was still Mong. So the English spelling froze the older Chinese name, and kept it after the Chinese moved on. Today the sign at the station says 旺角 above and Mong Kok below — two different names for the same corner, seventy years apart, mounted on the same board.
Hong Kong does this constantly. The romanisation is not a translation of the Chinese beside it; it’s frequently an earlier draft of it, preserved because nobody had a reason to update the spelling when the characters changed.
And the upgrade turned out to be right. It is, now, an extremely prosperous corner — possibly the densest retail street in the world. The name was aspirational when it was chosen and became descriptive later, which is the best possible outcome for a piece of wishful naming.