If someone wants to take a picture in front of a memorial, let them. I come from a family of military background and have been to many war memorials on this continent and others. While these are places of respect, memory, and sadness people still take pictures smiling. Why? Because they are happy they went there. A picture is a picture and if someone isn't hurting someone, then no harm done. Me standing there as Umbreon and taking a picture will not hurt the soldiers who are being remembered or the people. I'm not having a photoshoot where I'm posing around all these different markers with goofy faces or poses, I'm simply taking a photo of me by the memorial in the clothes I happen to be wearing at the time.
Having a huge photoshoot there is one thing but someone taking pictures in a costume is another. If you think it's bad, don't do it yourself but don't rain on another's parade because you don't think it's right. I don't think it's right for people to shave their heads real short, stick some up in a pink Mohawk, and then dye the rest to look like pink cheetah spots and then walk around calling other people freaks but I know someone who did it. I don't think it's right for people to do half the things they do but they still do them and I can't make them do otherwise.
I would rather have someone take a picture in a costume so that they will remember and cherish it than in normal clothes which will end up in the picture getting forgotten or tossed away because it doesn't have more meaning. And what about a lolita outing? Are you saying those girls can't go to a memorial and take pictures because it was a part of their trip? They have to completely bypass that whole experience because you think it's rude? I have pictures of the inside of Anne Frank's house in Amsterdam, the Tomb of the Forgotten Soldier in France, and the American Military Cemetery in France. Am I rude to have those pictures? No. Would be being dressed in cosplay make it any different? No. Because I was there and I want to remember those moments.
How someone dresses isn't the problem, it's how people act.