Well the only reason Letizia looked half decent during the engagement period was because she worked in television and had professional stylists. She has professionals dressing her from the time she came to be known as the Crown Prince's girlfriend. Mary didn't have this professional assistance so you cannot compare the two.
Mary no doubt also had a stylist at her disposal for her engagement announcement and between the engagement to the wedding.
And I don't know how it is in Spain, but in Canada journalists and news anchors certainly don't have stylists. They dress themselves. Part of their salary might allocate for money for an on air wardrobe or the television station may have a deal with certain stores where their anchors get a good discount there, but they pick out their own clothes. The shop clerks might help in recommending pieces or outfits, but anyone can do that at stores of a certain class that they go into. As a real estate agent/lawyer/marketing exec or whatever the heck Mary claims to have done for a career in Australia, she certainly could afford nice clothes. It's not as if she was having to dress at Wal-Mart.