No baseball caps or fanny packs visible here, but you take the point I hope.
"When in Rome do as the Romans. Please don’t wear shorts, a fanny pack, sandals, and a baseball cap as they scream “ugly American.”
I would add only that the visually jarring look described above also has its British, Scandinavian, and German variants. To borrow a page from the late Nancy Reagan's book, "Just say no!"
Unless of course you are by a swimming pool, or on a beach somewhere with the aim of looking like a cooked, slightly inebriated lobster by day's end when you stagger back to your hotel for dinner and later clubbing with all of the usual tactless, ostentatious displays of lowbrow drunks off the leash away from home for a few days. Then, by all means, go right ahead.
For those interested in doing things in a more understated way, dressing with comfort in mind when you travel is fine. Especially if you are visiting a warm place this summer. However, do so in a way that is respectful of local culture or customs, and helps you blend into the scene without screaming "Clueless, loud 'merican idiots here just asking to have our pockets picked in some form! Come on over!"
It is entirely possible, on the other hand, to dress for comfort, yet you can also manage to project an image of polish, sophistication, grooming, and finesse. It simply requires a tiny bit more forethought, care, and consideration as you pack your bag before departure.
As a gentle reminder, please do avoid insisting to the locals that you are German simply because a great, great grandparent might have come originally from Stuttgart, you have just visited The Hofbrau Haus in Munich, and here is the t-shirt to prove it.