Should I bring a rain poncho?

Yes, but get a high-quality one. The cheap plastic ones sold on the street rip easily and will make you sweat profusely. A breathable, vented poncho is much better.

Will my clothes get moldy?

If you leave damp clothes in a sealed bag for more than 12 hours, yes. Keep a separate mesh laundry bag for wet items and wash them as soon as possible.

Packing for Monsoon Season in Southeast Asia

Prioritize high-quality waterproof gear and focus on synthetic fabrics that dry in under four hours. Leave your heavy cotton jeans and leather shoes at home, as they will never dry in the humidity and rain.

Use a dry bag as your daypack. Instead of a standard backpack, use a 20L roll-top dry bag. It keeps your camera, phone, and power bank completely sealed from torrential downpours while you are moving between transport.
Choose synthetic or merino wool. Pack only items made of quick-dry nylon, polyester, or merino wool. If you get caught in a storm, these fabrics will be wearable again after a few hours of air conditioning or light sun.
Waterproof your electronics. Use individual Ziploc bags or silicone sleeves for every electronic device. Humidity is the real killer in the tropics, even when it isn't raining.
Bring lightweight, non-slip footwear. Bring one pair of water-resistant hiking sandals (like Tevas or Chacos) and one pair of lightweight, synthetic flip-flops. Avoid sneakers, as they take days to dry if soaked.

Should I bring a rain poncho?: Yes, but get a high-quality one. The cheap plastic ones sold on the street rip easily and will make you sweat profusely. A breathable, vented poncho is much better.
Will my clothes get moldy?: If you leave damp clothes in a sealed bag for more than 12 hours, yes. Keep a separate mesh laundry bag for wet items and wash them as soon as possible.