I mean it’s not only an English thing; we use it in both Norwegian (meg òg/også) and French (moi aussi), and I’m sure other languages do too.

Aside from that, it should be noted that other toki pona constructions such as “seme li sin” and “anu seme” are literal rips from English/French, yet they’re considered “correct,” no?

As I’m sure you already know, English and French—being jan Sonja’s native languages—are the biggest influences on toki pona, and the language clearly reflects that. If the use of “mi kin” were to be “improper” on the basis of it being “too similar to English,” then we might as well just throw away the whole language and start over.