What has the US and UK governments' political leanings got to do with this?
Once again, the classic "I know what 60 million people think" delusion. Look at the responses on this thread to the costume so far, most of them aren't that bothered by it.
As I've said already, I'm sure that you'll find people bothered by someone taking the piss out of Diana, but out of all the people I know well enough, maybe one of them might be offended by that?
Think for a second about how many people currently living in the UK weren't even born before Diana died (or old enough to remember her existence, or old enough to have developed any attachment to her considering that they would only encounter her in the news from time to time), the amount of people who are outright anti-monarchy here, the amount of people who have no strong personal feelings about the monarchy, people who aren't fans of (rumoured) marital infidelity, people who got sick of hearing her through tabloid news on a regular basis, the fact that just because some people might be considered "pro monarchy" doesn't mean that they liked Diana, and the fact that she died SIXTEEN FREAKING YEARS AGO, basically leaves room for some Diana fanatics who still gobble up the now-thankfully-occasional tabloid story about her.