Usually, women who travel to Tunisia, are encouraged to dress decently and conservatively according to the islamic dress rules, such as, for example, shirts covering the shoulders, long skirts or pants, no jewelry or makeup.

Women who follow these recommendations will soon find out that many of the tunisian women wear shoulder-free shirts, sometimes  even short skirts, often striking jewelry and almost always makeup.

Fact is that at least many young tunisian women, especially those who are financially better off and live in the tourist areas and large cities, dress very much "western" and follow fashion from Italy and France.

In rural areas, though, (and these often start already at the border of  large cities and a few steps away from tourist zones) the traditonal and islamic dress codes are (yet still) predominant.

For tourists who stay mostly in the tourist zones and large cities, there is, therefore, hardly any reason to dress "covered".

If one is visiting rural areas and traditional districts in towns, some decent clothing is recommended, namely for women.

Such areas are, however, almost never the destination of organized excursions of travel agencies.
Even the excursions to "authentic traditional areas" always end up with people who are very much used to be visited every day by large numbers of tourists...

Satellite dishes are nowadays on each and every roof in Tunisia and all young people watch the channels with music videos and american series all the time.

So, the population is used to the western dress style - and their opinion is made up already, be it positive or negative. And an individuum will not really change such an opinion in just a few days and just by wearing "appropriate" clothes...

A remark at the end: One can often hear that European women are  "dressed like prostitutes" - but this is nonsense and just a protective claim, because tunisian prostitutes are on one hand only rarely out in the public and on the second hand they are rather dressed "normal" conservative to blend into the scene.

And at night, in discotheques ... well ... it would be quite hard for tourists to compete with the clothes (or non-clothes) of some of the tunisian women who go there.