Is is okay to call someone a fool?

Yes

Jesus often called people fools. (So it must be OK.)

Ye fools and blind. Matthew 23:17, 19

Ye fools. Luke 11:40

O fools, and slow of heart to believe. Luke 24:25

So did Paul.

Their foolish heart was darkened. Romans 1:21-22

Thou fool. 1 Corinthians 15:36

O foolish Galatians. Galatians 3:1

And Nephi.

Nephi said unto them: O ye fools, ye uncircumcised of heart, ye blind, and ye stiffnecked people. Helaman 9:21

Whosoever shall say, Thou fool, shall be in danger of hell fire. Matthew 5:22