Is there such a thing as a good loan? My mum is convinced that HDB loans are good loans due to the relative low interest rates vis a vis other type of bank loans?

There are good loans and bad loans.
Good loans gives you more opportunities and profits. Good loans free up your cash flow so that you can use the money to invest. Good loan if utlise properly can generates more returns rather than cost. For example, house loan (viewed as assets), education loans, business loan

Bad loans depletes your cash flow and give you losses. Worst when loans get snowballed. For example, credit card, business loans that makes ends meet/ not generating substantial profit.

Hopefully, this can change your perspective in deciding your repayment options. There is no definite good and bad. What matters is the decision that suits you

I've a friend who told me, that she repaid her home loan so fast that she said on hindsight she should take a bank loan, instead of a HDB loan. Because bank interest loan rates are quite low now, but X years down the road, it may not be. My parents' era is about 5-6%.

So it boils down whether you want to take this risk, what is your repayment time horizon, your job stability, and whether you will remember to do refinancing / repricing every time your current contract is nearly up.

HDB loan has its advantages. It gives you a peace of mind (you know how much you need to pay every month) and also, for bank loan, don't forget they can repossess your flat. For HDB, I'd think you can at least negotiate when it comes to that.