More often than not, it doesn't matter too much if the leather isn't perfectly smooth. Stropping, in the most basic sense, is kind of unique in that it can be done on most anything, for most uses.
Having said that, if your strop happens to be loaded with a very aggressive compound, I'd try to keep the strop surface as uniform as possible. Any abnormal, out-of-place hard 'bumps' or ridges in the strop material, when holding an aggressive compound, could result in uneven results, focusing pressure in small portions of an edge and therefore removing metal more aggressively in some portions and less-so in others. Some strops can remove metal very quickly, depending on the compound used and how heavily it's loaded, relative to the steel's wear resistance.
And if a strop is very bumpy, that can sometimes make it harder to feel flush contact consistently. It's sort of a confidence thing that can be a distraction, if you're still working on technique.
And in bare leather strops, it's the 'silicates' in the leather that can make some difference.
