Following fashion in Mexico

Yes and no. Stacked high heels and skirts have been popular, nope not wearing either one. I do wear shorts every day though, and fortunately they seem to be crowding out the skirts.
A small percentage of the older natives wear more native or traditional fabrics, but otherwise people wear what the U.S. wears including shirts with English sayings that they may or may not be able to understand.

Recently I had a two week hunt for jeans . The stretch skinny jean is apparently it, here and in the U.S. normal jeans are hard to find. So I am stuck with jeans that go on more like nylons than pants .

There are many stores with lots of clothes and they are pretty reasonable. but it's hard to find things that were not made in Asia. I tend to try and buy Mexican or U.S. I couldn't even find U.S. made Levis in the Levi store.
Most of the popular stores are represented. The same Very expensive stores can be found in Cancun that you would find in the U.S.  and then there is Amazon Mx.

In larger malls in town and in Merida there are Gap stores, Levi stores Sears etc, plus some European stores. I would say the prices are less than in the U.S. for the most part.

Shorts and sandals

I am a US citizen living in the Yucatan peninsula.  I have been here for ten years.  My observation is, for this area, Mexican men have no sense of style.  I have seen couples; the women are always impeccable, but the men are "raggedity.".  Perhaps it is the males personality that is more important than the façade...LOL  This far south, fashion trends, I surmise, lag behind the more trendy "north".  When I arrived in Ciudad del Carmen, CM, in 2007. "Betty Boop" was the rage.  That was pase in the US in 1985.

My observation has been that the younger men dress with more style. The older men don't seem to care so much, perhaps it is the cerveza, and then there is the dating thing. Most of the older men are married I think. Also many have dirty jobs.  Lots of construction .painting. and building going on. They dress better to go to the malls, or go out to eat, otherwise they don't seem to care that much.

The women often over dress in an uncomfortable , dangerous manner. I cringe when I see them trying to get on an escalator with 4" heels. The younger women seem to be wiseing up, because I am seeing more of them in sandals. It is also hot, often very hot, so comfort is very important. I usually wear only sandals and shorts with a t-shirt.

Hi, my wife and I will be moving permanently in a few months. We will be staying in La Peñita de Jaltemba which is a small town 80km from Puerto Vallarta. In town, there is a few small stores that carry enough options (shoes, shorts, pants, etc...) to be in style.

When we will be in Canada visiting, we will buy items not available in Mexico.

Also, there are major strores like Liverpool and Lans were you can buy the latest fashion but need to look for special  (Buen fin in November) or be ready to pay muchos pesos.

Overall, there is no problem or concern for us. GyC.

I generally wear shorts, skirts with tank or short sleeved shirts or tshirt or summer dresses. Dry wick or cotton fabrics are best as anything synthetic, long sleeved, full leg or tights are just to hot for enjoying Cancun and Yucatan weather.   Beach wear is bathing suits and breezy coverups. I seem to be always late lighter wear than most Mexicans or expats living there full time. Sandals and walking shoes are a must. I get most of my clothes locally in Cancun. The Nike Outlet store and the Sketchers store in the Plaza Outlet mall is great place to shop, and the lingerie and Sangara stores are great finds for swim suits, bras and panties.