To be fair that wasn't the intention of the match at the time of the marriage Francis' eldest son was still alive - so Catherine was not expected to be Queen of France just Duchess of Orleans - the dauphin died three years after Catherine's marriage - however it was certainly considered that she was of relatively low birth for a French prince. She was also the niece of the Pope and her late mother had been French and heiress of the Tour d'Auvergne family - hence the attraction to the match. Noticeably he didn't offer the Pope the dauphin for her.Who else was there besides sons of Francis and Charles?
And did they even have a lot of other options? Francis' successor married Catherine de Medici, who ranked FAR lower than a princess of England