You cant really find mule skins anymore, at least if you ask commercial conga makers, LP, Pearl, etc... they use mainly thai water boffalo, also calf skins are popular but mule isnt, i think its because its hard to get in large quantities,
(since mules dont breed).
i would think that small time conga makers,(hand made), could use mule, but i also think its more expensive than other skins.
on smaller drums ,(requinto size), goat skin sounds good, it gives a nice high sound, but on larger ones it isnt thick enought to produce a good sound.