The west coast should be OK during the period you have in mind. Although west coast diving cannot compare to the quality the east coast offers, conditions should be good. I have only dived in Pulau Payar (marine park off Langkawi) and conditions and diving were good but not necessarily worth traveling around the world for. On the other hand, this was a nice addition to a holiday which had not been designed to be dive-specific.
I guess that if the main purpose for travel is diving in Malaysia, then the timing is a bit off - if, on the other hand, the main reason is to see parts of the country, and get some diving in while there, it can certainly be done.