Yes, I remember the “mission accomplished” banner as being on an aircraft carrier. But I don’t understand why you see the mention of it as a leftist talking point.
In his speech there, Bush II told the country that major combat operations in Iraq were over. Saddam was deposed. He was specific. So it’s fair to conclude that the banner was connected to the war itself.
Yet, you might have a point. Some sources say the banner was the idea of the ship’s crew, and referred to the ship. But others say the idea came directly from the White House, and referred to the war. Lots of contradictory info there.
Bad optics regardless, as you noted.
Democrats and the media made a big deal over it at the time, especially Howard Dean and John Kerry, because they were running for president. At that time, Bush’s approval rating was in the 65% to 70% range, so they were pretty worried and anxious to try and make a big deal out of anything to try and discredit him.