It is to a certain extend, but the point was, American tourists are usually easy targets for pickpockets because they stand out. This goes for all the big cities in Europe. Pickpockets are everywhere.
But isn't this true of any tourist-who-looks-like-a-tourist? American or otherwise?
I'm neither American nor European, so I don't really have a dog in this fight. But even so, some of the comments in this thread, however well intended, have struck me as elitist, snobbish, possibly offensive, and certainly stereotypical.
The fact is, most tourists of any nationality tend to stand out merely because they are visiting tourist areas, often are not speaking the local language, don't look like they're in a hurry to get anywhere, may be carrying a camera / backpack or actively taking pictures, perhaps are disoriented or look confused, and yes, may be dressed more casually than locals who are going about their daily business. Much of this in unavoidable, especially on a short stop when absorbing the local culture in any meaningful way is not really practical, and seeing the highlights is probably a priority. And it's just as true of European tourists in the US as it is of American tourists in Europe.
I doubt that pickpockets really discriminate much based on nationality, even if they can identify it.
I actually took OPs original question to mean attire while
on the cruise ship, not necessarily while in port. And for what it's worth, on the couple of European
DCL cruises I've done, I've never noticed much of a difference. Maybe partly because DCL's clientele are still heavily American, even in Europe.