It is OK.
The fool has said in his heart, “There is no God.”
Psalm 14:1, 53:1 NASB
He who trusts in his own heart is a fool,
Proverbs 28:26 NASB
17 You fools and blind men! Which is more important, the gold or the temple that sanctified the gold?
18 And, ‘Whoever swears by the altar, that is nothing, but whoever swears by the offering on it, he is obligated.’
19 You blind men, which is more important, the offering, or the altar that sanctifies the offering?
Matthew 23:17-19 NASB
(More: Luke 11:40, 24:25, Romans 1:21-22, 1 Corinthians 15:36, Galatians 3:1)
It is not OK.
But I say to you that everyone who is angry with his brother shall be guilty before the court; and whoever says to his brother, ‘You good-for-nothing,’ shall be guilty before the supreme court; and whoever says, ‘You fool,’ shall be guilty enough to go into the fiery hell [KJV shall be in danger of hell fire].
Matthew 5:22 NASB
Intro
Several times there is a warning in the Bible to not be a fool or to become a fool. However Jesus forbade to call another a fool. Isn’t that a clear contradiction? It becomes only worse when we hear Jesus say to the Pharisees “You fools and blind men!”
Example
Once—when I was joking—my little granddaughter said to me “Mad grandpa!” and she laughed. Then I said to her “No, we don’t say mad to each other here. You may say that I am acting madly or foolishly, but not that I actually am mad or a fool. I will never say to you ‘mad’ or ‘fool’, but that you are doing foolishly sometimes, or madly.” Her eyes told me that she had understood the difference. We should avoid cursing each other with negative labels with ineradicable power. By that we keep our inner self free and useful to be creative people in life. Not only they who receive such a negative label, but also one who gives it run the risk of becoming a contorted person.
Conclusion
In the same way we see in the Bible that the word fool/foolish is related with warnings to act wisely instead of foolishly; not to be fools but to become wise. The differences are delicate but always clearly avoiding pinning down a certain individual to a final negative judgement of being a fool.
The list
Psalm 14:1 Here it is said in a general warning, the fool as a group (collectively).
Proverbs 28:26 Idem
Matthew 23:17 Idem
Matthew 5:22 Prohibition to label any individual as a fool.
No Bible Contradiction