The American Civil War and how both sides have honored their dead have been relatively controversial. Especially with the rise of the Lost Cause around the turn of the century.
In other countries around the world, have their been equally controversial celebrations or memorializations of the losing side? Off the top of my head I am thinking of the Irish insurrections that have occurred over the years but i would be curious if anyone had other insight.
The "good old cause" from the English Civil Wars fits the bill.
The good old cause was short hand for the democratic and republican political principles of the Leveller faction of the Independents in Parliament. These beliefs were suppressed after 1649; first by Cromwell and then by the restored Stuarts. Their color was sea green and their devices or field signs were a green ribbon or a sprig of rosemary. Thomas Paine's Common Sense echoes many common Leveller themes.
See: Free-Born John the Biography of John Lilburne by Pauline Gregg; and Puritanism and Revolution by Christopher Hill.
In Ireland, wearing the poppy on Armistice Day has been, historically, controversial, although it's become less so lately. In the UK people wear poppies in November on their lapel to commemorate the dead of the world wars and other conflicts since. Thousands of Irish have also fought and died in the British Army, but after Irish independence, they have been overlooked in favor of Irish independence fighters. Wearing the poppy in Ireland or commemorating British war dead, became correlated with Protestantism or Unionism, and still is to some degree, although lately tempers have cooled over the issue.