kama means come, pona means well. So kama pona means well come or welcome. That seems to be to be a euro-centric translation. But if the words are sematic primatives, and if we don’t want to project our language onto toki pona, we’d translate it as, “You did a fine job of arriving” or something like that. If we were to translate from Klingon, it would from Qapla‘ to “pona la sina pini”, meaning something like, “you have finished well” or ”you will succeed”.
Here’s some alternatives, pick depending on what you really mean:
tan seme la sina kama. Why are you here?
pona la mi lukin e sina. It’s good to see you. (Or I see you clearly)
o sina: sina wili moku telo? Would you like something to drink?
o sina: sina li nanpa tu–a. You again, *sigh*.
I’m not sure about the vocative, I might change that. I’m also not sure if lukin takes the accusative or if it has to be used with another prepositional phrase.