Dressign up as LatvianSS is not celebrating the killings committed by the Germans.
It is celebrating the occupation of Latvia by Germany, and the subservience of Latvian home guard to Germany. If we were to take my example of the Boston Irish dressing up in sheets a bit further, and have them doing this for the St. Paddy’s Day parade, and in gratitude for all the help that Robert Byrd and David Duke gave them against busing and affirmative action, would dancing the Irish Jig and getting plastered somehow make it about being Irish? That’s aside from the killings, which btw the locals took part in under the leadership of the SS. Latvia was unfortunately caught between the devil and the deep blue sea, so they chose German subservience over Russian subservience. The Slovaks did something similar. It’s not a pleasant thing, but it happened.
Do you celebrate that subservience 70 years later? I’m not Latvian, but if I were I would have nothing to do with commemorating my country’s occupation, especially not wearing a hated uniform like that.