Chance of seeing sloth bears or other rare mammals in Sri Lanka?

Sloth bears are very shy and mostly nocturnal - your best chance is in Yala or Wasgamuwa, but even then sightings are rare and usually at dawn/dusk. Pangolins are almost never seen by tourists. For better odds of interesting mammals, focus on the national parks at the right times and go with experienced naturalist guides. Most visitors see monkeys, mongooses, and various deer species easily.