Our priest, who is quite strict about modesty, has spoken from the pulpit about modesty on quite a few occasions. He said he finds it edifying to see women wear a tee shirt underneath a dress or shirt that doesn't meet the modesty standards by itself. So in this case, it is more of an "underwear" type of garment.
In the book, "Dressing With Dignity", Colleen Hammond discourages tee shirts with wording on them. It may not be outright immodest, leading others to sin, but it goes against the feminine nature of not trying to advertise, draw attention to oneself. Also, with certain prints, one may ask where does the print on the shirt lead the eye? I don't have the book in front of me, but she gives several standards for determining if a shirt is properly modest in the way it fits.