It's been said many times Singapore is clean only because there are strict laws about being fined for littering, and also because there are cleaners to quickly clear whatever mess is being created anywhere. Minus off these 2, we're in for trouble. For example, with the ban on chewing gums, how can this tiny red dot not be clean? Bring back chewing gums, and we shall see.
So the question I feel is not so much about "Is Singapore clean?" but rather "Do people in Singapore practice good habits to KEEP Singapore clean?" We all know the answer is a firm No.
I guess the writer of this post just wanted to highlight that even a 3rd world country like Thailand is actually GENUINELY cleaner than Singapore. To that, I agree. So my own opinion is that it's been over-hyped that Singapore is clean. I get some goosebumps whenever I'm overseas and tell people I'm from Singapore, and the foreigners go like: Oh wow... I've heard Singapore is very clean. Usually I give them a half smile and quickly compliment their own country in other areas which Singapore lacks.
And I don't think it's right to "blame the parents" entirely as some fella or fellas say here. That's only part of the picture. I think it's more correct to say it's the general culture here. It's been like that for decades. Moreover, blaming the parents won't solve the problem. From 1980's till now, I've seen plates and half-eaten dishes strewn all over the table after the customers of a hawker center or restaurant leave the table.
You know it when it's crowded at a hawker center. You wait for the cleaner to come while you stare at the table filled with spilled soya sauce/gravy, prawn shells, noodles spilling out of the plates, and the one that I cannot understand and totally feel squeamish about is: Tissue paper with possibly mucus in it floating in a bowl of soup!!! Some jokingly say they look like dumplings. Why can't these inconsiderate people just take their freakin' soiled tissue paper and throw it away at the nearest bin?