I noticed that the tag smoothness=p is basically only used in Yaoundé. See https://overpass-turbo.eu/s/1jLm.

What is its meaning?

To help navigation apps, it would probably be better to use a conventional tag listed in the wiki: https://wiki.openstreetmap.org/wiki/Key:smoothness

If surface=p can be translated to one of the conventional values, it might be possible to replace this tag via an automated edit.