The summer season is good because it allows you to visit the East Coast islands (Perhentian, Redang, Tengol etc.). These islands close in October/November and reopen in February/March (you can still go to Langkawi, which is on the west coast). The places where you intend to visit (Taman Negara, KL, Cameron Highlands) will not be affected by that monsoon season so do feel free to come here. You tend to see more Australian tourists in during the Christmas period.
However, do take note that the Christmas season is considered peak period in places like Langkawi. During this time, hotels charge ‘rack rates’ which means The Westin Langkawi (as an example) will cost RM1200 per night for the cheapest room instead of around RM650 (online rate outside this period) for the same room. And yes, all the 5-star hotels do get booked up and will not accept a cheaper rate during this time. The increase in cheaper places (chalets, guesthouses etc.) should be minimal.