Can I give a slightly different view? When we arrived at the Cathedral last week for our first ever pilgrim mass I loved the fact that so many people there had clearly, as we had, walked straight into the city from their camino and hardly stopped to dust the sand from their feet, so to speak. There were shorts, worn out trainers, hiking boots, scruffy Tshirts, bare shoulders, unwashed hair - and none of it felt in the least disrespectful. Exactly the opposite to me - the Cathedral is a place of pilgrimage, and the dusty, unwashed pilgrims, I felt, were welcomed with open arms to their spiritual 'home'.
It would be different at a 'normal' mass on a Sunday morning, but for me, to be in communion with fellow pilgrims at a mass in which we were specifically welcomed and blessed, was a powerful and incredibly moving experience and it would have felt different if I had first gone to my accommodation/washed/changed (indeed it did feel different when I went back to the pilgrim mass the following day, looking much tidier but already starting to feel less like a pilgrim).
Far more disrespect was shown, I felt, by those (peregrinos and, far more, tourists) who insisted on taking photos on their mobile phones throughout the mass. And most of all as we went up for the Eucharist - including one chap who was spoken to by the security man and still carried on videoing the whole service
I suspect neither God nor St James will mind what you are wearing
@francesh17 - they will just welcome you as a committed peregrino with a good heart. Buen camino!