The days in tropical Malaysia are usually hot and humid. It may rain cats and dogs every few days in most months but its still hot and humid most of the time with 27 degrees Celsius the coldest during the day with an average of 32-34degrees Celsius most of the time.
And on weekends, everyone dresses down. This is Yours Truly at the Pavillion Shopping Mall in Kuala Lumpur on a Sunday. Even here most are in slippers and shorts, sandals and jeans. I still chose ot wear a short sleeve batik shirt, cotton trousers (in the correct length) and a pair of Italian moccasin loafers (a comfortable pair that I've owned for over seven years and still going strong). Also note that in these parts, a Batik shirt is considered as part of formal wear.
On other less formal locations like the neighbourhood mamak stall I still wear jeans and a polo t-shirt or a round necked tucked in. With shoes - boat shoes or moccasin loafers. No flipflops or sandals.
Yes. Dress slightly better than others. Even on a Sunday.This is in case you need to be somewhere that requires you to be better dressed in a jiffy and you already are in a presentable state But I also suggest keeping things basic as you must realize that Malaysia is hot and humid. Do not over do things. A shirt, a pair of pants and a nice pair of shoes would actually do just fine.