I do not believe that all crusaders from 1095 to 1099 were wearing mail armour when marching or fighting towards Jerusalem.

By this time, iron and steel weapons and armour were very costly to manufacture. They weren't as costly as they had been the centuries prior, as stated by the pricings found in some Carolingian capitularies such as the Capitulare Missorum of 802-803, but were still an enormous cost which could be afforded by few.
Namely, these few would have been landed aristocrats or scions of wealthy families, the kind of people who could and would comprise the mounted knights following the like of Bohemond of Hauteville, prince of Taranto, Godefroi of Boullion duke of Lorraine. This would have been the case since by this time, most families of aristocrats were facing the serious problem of shrinking inheritances due to what we can call overcrowding.

Foot soldiers which accompanied these armies would wear mail armour often, if at all. For the most part would have worn some kind of protection of the textile variety of body armour. These, named gambesons with a word deriving from Old French, were padded jackets stuffed with wool or many layers of linen and crompressed through sewing (someone has speculated in a sort of primitive quilting), resulting in a way cheaper protection that could offer reasonable protection against slashing and piercing, even more so against blunt damage. This kind of protection is way more common in the XII and XIII centuries onwards, but there is consesus that it may have existed as far back as the mid IX century.

Unfortunately, archaeological evidence of this sort are very difficult to come by due to their inherent weakness against decaying, if compared to other findings in graves of large pieces of iron mail armour.