Why did God Almighty forgive Jonah after his supplication, even though he was blameworthy, and not forgive Pharaoh, who was also blameworthy, despite Pharaoh's saying: "I believe in that which the Children of Israel believe in," when both were in a critical situation?

Allah does not wrong people in the least, but people wrong themselves. Yunus (peace be upon him) was a noble prophet who did not commit anything deserving punishment, whereas Pharaoh was a mighty tyrant who defied his Lord. Allah described Yunus as "blameworthy" because he departed without Allah's command. As for Pharaoh, he was blameworthy due to his claim of divinity and his denial of the Messenger. Pharaoh's repentance was not accepted because it occurred at the moment of drowning, which is a coerced belief that is of no benefit when facing death. In contrast, Yunus repented and acknowledged his fault, so Allah saved him. Equating Pharaoh's situation with that of Yunus is a flawed comparison, for the blame and punishment of a disbeliever differ from that of a disobedient believer, especially if he is a prophet.