@twinklecake I recall it was actually hyperthermia, due to a rocket not detaching as intended. If all had gone to plan, Laika would have been given a poisoned meal as euthanasia so she wouldn't suffocate to death as oxygen ran out, but instead she died of overheating.
One nice thought (at least I think so) is that, because her capsule disintegrated and burned up upon reentry, her ashes would have been scattered across the entire planet by high altitude air currents. Appropriate, having been the first creature to see it all from orbit.