What does Kino’s straw hat symbolize? Why is he dressed this way? Why does he not dress like this all the time?

The hat represents status and a new sense of worth.

Kino put on his large straw hat and felt it with his hand to see that it was properly placed, not on the back or side of his head, like a rash, unmarried, irresponsible man, and not flat as an elder would wear it, but tilted a little forward to show aggressiveness and seriousness and vigor. There is a great deal to be seen in the tilt of a hat on a man.