What does “yet don’t look too good, nor talk too wise” mean in the poem "If" by Rudyard Kipling?

Exaclty. This poem is all about balance and perfecting one's attributes. "Don't look too good" means you're not putting on airs, not going to great lengths to outshine everyone. "Nor talk too wise" is along the same lines. It means you don't have prove your intelligence at every moment...you don't need to talk down to anyone. So basically, the poem is saying if you can do all those other things, without becoming egotistical, then you've made it.