Why does "everyone" in this book totally fall in love with Diana and find her gorgeous, charming and interesting when the character isn't described that way at all! Who likes snarky American sarcasm-speak that Diana uses all the time, it's really offputting. Who likes a woman who drools over her man at every opportunity.
Martha Like Sarah-kate said, one key point to remember is that the description we get of Diana as a mousy-haired uninteresting person is from her own viewpoint. As Chris remarks in Book of Life, in actual fact, even before coming into her powers, Diana couldn't even walk across the quad without people noticing her. (Also, sarcasm-speak? Were you reading the same book as the rest of us?)
Sarah-kate Magic and power can be alluring. It's stated repeatedly that people and creatures 'notice' her due to her power. A lot of the time you can be drawn to a person by more than just their looks. As the description of her (if I recall) is from her own perspective it's understandable that she'd come across as a plain Jane type; you're viewing her from her own eyes.
M Not everyone. Diana the typical pseudo-heroine where she is left intentionally bland and somewhat annoying so that unintelligent readers can easily relate to her (as a reflection of themselves) and live vicariously through the character when she meets the typical rich handsome guy over her league. it sells books at the cost of propegating the plain jane gets the rich hot guy dillusion.