What should I wear in Mexico City next week given that it's Summer season but 77/53 degrees?

The only thing that would make you stand out is something expensive like what you describe. There are so many people in Mexico City, you see all types of styles. Avoid jewelry.

Wear a light sweater that you feel comfortable carrying around. The mornings and nights can be chilly. Don’t forget your umbrella.

Down in the central part of the city it will be warmer than if you are higher up in the mountains, I am assuming you are staying downtown? Bring a cardigan you can easily take on and off, especially in the evening as @bob_ suggested. Stay on the beaten path, Mexico City can be dangerous like most large cities, use your city sense. Know your surrounding and walk with purpose. Leave expensive jewelry at home is also great advice, plus I would leave t-shirts that imply you are American at home. That is if you are American.

A mask. The air pollution is beyond horrible.

The pollution is bad. And, if you get altitude sickness plan light touring/walking for the first couple of days, save long days for the middle of the trip. I never had any problem with the altitude, but some do.

Avoid black clothes and jewelry

Avoid white pants, like pretty much any city.

Air pollution is NOT beyond horrible. You do NOT need a mask. They run freaking marathons in Mexico City, for crying out loud. It’s not the Alps, but it’s not something to worry about.

Did it sound like we were saying there is so much pollution you can’t breath? @bob_ is right, you will be able to breath, don’t worry. You will notice it more air pollution than most North American cities though.