Maybe to some other contexts, but when compared to other contexts, it’s not high at all. Singapore has rather average total incarceration rate, globally. This is already a good look for them: most of the crime in developed world is committed in cities, especially large ones, and as practically entire population of Singapore lives in a huge city, one would expect its criminality to be significantly higher than global average. For comparison, if you subtracted crime committed in its large cities, United States would be extraordinary safe place, with very low resulting incarceration.
Now, if drug offenders make up two thirds of its already low (when controlled for urbanization) prison population, it means that its population of non-drug offenders is extraordinarily low. This means, for example, that its entire non-drug population is (per capita) half as big as those of New York City jails, which only contain those in pretrial detainment, and serving short sentences under a year (and not even all of them at that), and do not detain those NYC criminals serving longer sentences — and NYC in recent years has been known as rather safe big city, with low crime by US standard.
So, Singapore is a place with extraordinarily low crime, and extraordinarily low incarceration of non-drug offenders, and low incarceration overall (when controlling for urbanization). And that means that Singapore drug policy doesn’t work. Okay, I guess.