Across the whole of Malaysia the weather patterns have changed.
That said, the weather is fairly consistent with little change between the wet (monsoon, as some might call it) and not wet (or dry) season...
Hot at about 30c to 38c
Humid at about 95% ++ humidity
Rain - it might rain - it might not rain. If it does rain, it is likely to be a heavy shower or thunder storm in the late afternoon or evening.
Usually only last a couple of hours.
Sometimes, it might rain all day - but not very often.
Clothing - light weight and comfortable. Bring an umbrella & wear sandals.
Borneo and The East Coast monsoon is late November to mid February. This is no longer a reliable weather pattern and the above info applies. This weather system also effects southern states like JB and Melaka.
The West of the Peninsular tends to get more rain between August and October. Your location makes a difference: Langkawi is a bit wetter August and September.
Penang is wetter in September/October.
Once again, this is no longer predictable.
The problem with the tropics is the heat and humidity can create rain clouds and thunder storms at any time. You might be lying on a beach whilst 3 k away it is raining heavily.
Bottom line - even if it rains you will have FUN.