Does God forgive those who improve their prayers out of showing off to people? And does the Messenger of God’s (peace be upon him) fear of this act, even more than the tribulation of the Dajjal, mean that it is unforgivable?

Hypocrisy (Riya') in acts of worship is originally a minor polytheism (shirk asghar), unless the hypocrisy is pure, meaning the hypocrite prays solely for the sake of creatures without any regard for the Creator. This is hardly something a believer would do, and it constitutes major polytheism (shirk akbar).

The Hadith: "Hidden polytheism (shirk al-khafi) is when a man stands to pray and beautifies his prayer because he sees someone looking at him," does not imply that hypocrisy is major polytheism in all cases.

One who repents from hypocrisy has their sin forgiven, for repentance erases sins, even both types of polytheism.

Whoever dies upon hypocrisy, which is a minor polytheism, is under the Will of Allah: if He wills, He forgives him, and if He wills, He punishes him. Some scholars are of the opinion that minor polytheism is not forgiven, just like major polytheism, except that its perpetrator will not abide eternally in Hellfire.