2 Kings 9 NIV
31 As Jehu entered the gate, she asked, “Have you come in peace, you Zimri, you murderer of your master?”[e]
Why did Jezebel call Jehu by the name Zimri or was Zimri the other name for Jehu?
The hermeneutic here is to look back to "first mention": Who was King Zimri?
Jezebel was probably likening Jehu to King Zimri. This happens in today's English also, calling someone by someone else's name to suggest a similarity. It happens in many nations and in many languages.
Zimri was king only seven days, then died—only a few chapters before this event of 2 Kings 9:31.
1 Kings 16:15, 18 (NASB)
15 In the twenty-seventh year of Asa king of Judah, Zimri reigned seven days at Tirzah... 18 When Zimri saw that the city was taken, he went into the citadel of the king’s house and burned the king’s house over him with fire, and died,
Interestingly, Jezebel was an unabashed murderer (1 Kings 19:1-2; 21:1-16). And, Jehu had just become king a few verses before in that same chapter (2 Kings 9:11-13).
By her reputation as a murderer, and speaking to newly-anointed King Jehu, when she refers to King Jehu by the name of a recent king who died after only one week on the throne, the most obvious and immediate meaning from Jehu's perspective should be that she was threatening to have him murdered, just as she has openly done before.
Further study
This is an old question, as demonstrated here: 2 Kings 9:31. Jehu as Zimri
The statement itself looks metaphoric and on closer look, one will see the similarities between Zimri and Jehu and this was not just a physical one but a matter of spirits and assignments.
In 1Kings 16:9, we are introduced to Zimri, the servant of Elah the son of Basha.
9 And his servant Zimri, captain of half his chariots, conspired against him, as he was in Tirzah, drinking himself drunk in the house of Arza steward of his house in Tirzah. 10 And Zimri went in and smote him, and killed him, in the twenty and seventh year of Asa king of Judah, and reigned in his stead. 11 And it came to pass, when he began to reign, as soon as he sat on his throne, that he slew all the house of Baasha: he left him not one that pisseth against a wall, neither of his kinsfolks, nor of his friends. 12 Thus did Zimri destroy all the house of Baasha, according to the word of the Lord, which he spake against Baasha by Jehu the prophet,
Jezebel was a sorceress. The cause of death of her husband couldn't have been a mystery to her neither was the perpetrator of that act unknown to her. Jehu later acknowledges the part he played in the death of his master as seen in 2kings 10:9.
9 And it came to pass in the morning, that he went out, and stood, and said to all the people, Ye be righteous: behold, I conspired against my master, and slew him: but who slew all these?
Jezebel calling Jehu Zimri was not only metaphoric but a confirmation of Jehu's assignment. Remember that just like Zimri who slayed all that was in any way connected to Basha, so also did Jehu slay all that was connected to Ahab.
It can also be said that Jezebel was invariably saying that just like Zimri who died just a little after he became King, Jehu would also meet a similar end.