First of all, I don’t think I would have even batted an eye to begin with, but…since the OP asked, I was giving my opinion. It does seem to me that I had read somewhere, in the past, that spaghetti straps and sleeveless tops and dresses were not a good choice to wear to Mass.
I wasn’t pretending to have all the answers either. I sighted a source from the Vatican, and now I have another one.
Pilgrims to the Vatican may have their own stories of disappointment or near misses to tell.
On sweltering summer days in Rome, scores of visitors are turned away from St. Peter’s Basilica or the Vatican Museums because they’re not dressed decently.
Transparent clothing, halter and tank tops,
spaghetti-straps, and shorts or skirts that don’t reach the knees** are strictly forbidden**.
(my bold)
source: Catholic News Service
catholicnews.com/data/stories/cns/0903275.htm