Is it a good time to invest in Nikko AM SG STI ETF?

It is definitely a good time now to invest for long term. In terms of ETF, I perfer to invest using roboadvisor for diversification and flexibility to add funds.

If you intend to invest long term. Yes.

And I would recommend you to setup a RSP for monthly investment to DCA. It would be useful in this period of pandemic where there's a lot of uncertainty. If the ETF drops further in the next few months, thru RSP will allow you to buy more shares and lower your cost per unit.