The "Olsen's" were a composite of two separate families encountered by the real Ingalls family. The characterisation of "Nels Olsen" as likeable and sympathetic was a fiction, created for the TV series.
Half the time, you don't even SMELL like a girl! You're either sweaty, or you stink of fish!
Laura: Well... I sweat a lot and I fish a lot!
The series often preached "acceptance" and "integration" of the downtrodden and of those who are marginally different than the rest (such as the dwarf, the wild boy etc.) yet those characters who---invariably---would be accepted in the Walnut Grove community, would subsequently never be heard of again, for the rest of the series, as if they'd never existed, let alone moved to Walnut Grove. In a way, this defeated the whole "social tolerance" progressive theme of the series.