Was skimming thru and this thread does look like some explanation needed on the perception of "poor" local Mandarin.
First, Singapore is not part of China and it would be ridiculous to assume that locals have to speak like Chinese Mainlanders. Just like the Americans and the Brits, they speak English with different accents too.
From Chinese mainlander's perspective, they would consider their Mandarin to be de-facto standard, and Singapore's Mandarin version is admittedly non-Standard, but ditto for Taiwanese Mandarin as well, but foreigners are still flocking to Taiwan to learn Chinese. From a standard point of view, Singapore Mandarin fares behind both Taiwan and China, this is because of a few reasons. One of them is that early Chinese migrants to Singapore were from Southern China, and the Southern dialects are vastly different even amongst each other, not to mention its differences when compared to Northern dialects which by and large are quite similar, and the Standard Mandarin in China is based on the phonology of Beijing dialect, which is one of the Northern dialects. To facilitate a common language amongst the different Chinese dialect groups in Singapore, Chinese is adopted as the "common language" but you have some of the non-standard grammar and vocabulary from Southern dialects that crept into local Mandarin. For locals, our unique Mandarin is synonymous with our local identity but unfortunately our local Mandarin might not be understood by Chinese mainlander.
But for local tutor, if they are trained in Standard Mandarin and is experienced in teaching, they are just as good as Mainlander or Taiwanese teacher. In fact, they would probably understand your difficulty in learning Mandarin as a foreign language since most of them are conversant with English and Mandarin. The other fact to know is that (based on my years of teaching experiences), if you are learning Mandarin during your latter years, perfecting tones are extremely difficult, and it is likely you speak like Singaporeans as well.