It depends what a crowded trail means to you. I did 3 stages of it on different years, and all were actually quite similar. The northern part is more tourists, the southern is more Swedes since it's closer to get to from Stockholm. Some days you'd only pass a handful of people, some days you'll see 20 people. It is more crowded by big cabins (fjällstation) like Kebnekaise, Saltoluokta. I found that generally people spread out quite a bit (it's still in Sweden, so people like personal space). So it didn't feel crowded to me. Just time it to not walk during Fjällräven classic.