Is hiking in Sri Lanka's mountainous regions prone to muddy conditions or landslides?

Here's the answer to your question: It's highly likely, but it entirely depends on when and where you go.

Simply put, the hiking experience and weather in Sri Lanka's highlands are directly linked.

The key factor is the "rainy season":

Sri Lanka has two main rainy seasons (monsoon seasons) that affect different regions. For the central highlands, where hikers most frequently go, such as Ella, Nuwara Eliya, and Horton Plains, the Southwest Monsoon has the biggest impact, roughly from May to September each year.

During the rainy season (May-September): If you go hiking during this period, encountering mud is commonplace. Mountain trails become slippery, muddy, and some sections might even feel like wading through thick sludge. Continuous heavy rain also significantly increases the risk of landslides. Local news occasionally reports landslides or road closures due to heavy downpours. So, if you go during this time, not only will your hiking experience be diminished, but the safety risks will also be much higher.
During the dry season (December-March): This is the golden time for hiking in the highlands! The weather is clear, temperatures are comfortable, and the trails are dry and easy to navigate. You generally won't have to worry about mud, and the risk of landslides is at its lowest. The scenery is also at its best, with expansive views, blue skies, and white clouds complementing the green tea plantations, making for excellent photos.
Transition seasons (April, October, November): The weather during these months can be quite "unpredictable"; it might be sunny and bright, or a sudden downpour could occur. Therefore, the chance of encountering mud is fifty-fifty, so it's crucial to pay close attention to the weather forecast before you set out.

Let's talk about different trail conditions:

Popular, well-established routes: Trails like Little Adam's Peak, Ella Rock, or World's End in Horton Plains are relatively better maintained because they see more foot traffic. However, after rain, the dirt sections can still become very slippery, especially the latter part of Ella Rock, which requires using both hands and feet, and can be quite messy when muddy.
More rugged routes: If you plan to visit more secluded, pristine areas, such as the Knuckles Range, the weather requirements are even higher. The trails in these places are already challenging, and once it rains, the risk of mud and landslides skyrockets. It's strongly recommended to go during the dry season and with a local guide.

To summarize my advice for you:

Preferred time: Try to schedule your hiking plans between December and March.
Essential gear: Regardless of the season, always wear a pair of hiking shoes with good grip! This is not a joke; regular athletic shoes covered in water and mud will make you question your life choices.
Monitor the weather: Before you leave and on the day of your hike, always check the local weather forecast. If heavy rain is predicted, decisively abandon or change your plans; safety comes first.
Consult locals: Ask your hotel owner or local guides; they know the latest trail conditions best.
Manage expectations: Even in the dry season, mountain weather can be variable, and occasional showers might make trails a bit slippery. Just slow down and walk carefully.

Overall, if you choose the right time, the hiking experience in Sri Lanka is truly wonderful. If you choose the wrong time, it might turn into a "mud survival" adventure.