"Why is the baby dirty?"
Translation:Mbona mtoto ni mchafu?
The only difference between kwa nini and mbona is that mbona expresses surprise as well. You could translate it as “why”, “how come” or “what ... for”, just like you could with kwa nini. Mbona is a little bit more in the direction of “why the f**k/hell”, but not as strong and not offensive, maybe a bit like “why on Earth” if that’s something you say. In this sentence, you’re not only neutrally asking for the reason but also expressing that you’re surprised that the baby is dirty and probably also implying that the baby shouldn’t be dirty.