The Queen wears gowns and tiaras almost always and refers to this as her working uniform. Eileen prefers smart casual trousers and tops especially when flying. This has caused some disapproval but the Kingdom generally can see Eileen does have a point here. Tutus are no good to anybody when caught in a cross wind!
Eileen can and did dress up during her time in the Palace, which, for ceremonial occasions, she doesn’t mind, being glad to play her part. Where Eileen resents is when the Queen expects her cousin to “doll up” for private dinners and things like that. Eileen has no problem with the Queen laying down the law, it is the monarch’s job, but to tell her own cousin how to dress is a role too far. Eileen never looks scruffy. She has far too much pride for that and, for Eileen, that should be enough to satisfy the Queen.
It isn't, of course, given there's so much in Eileen's behaviour and attitude the Queen feels she has cause to be very dissatisfied about. The Queen herself always looks immaculate. She feels she owes that to her public. Choosing what to wear is one of the highlights of her day. She loves her full length gowns. She loves bright colours (though she probably isn't going to wear the yellow taffeta ballgown again after Eileen accused her of looking like a walking daffodil. Absolutely nobody else would have dared make that criticism but it was only in the privacy of the royal bedroom when the Queen took a good hard look at herself in the mirror, she realised Eileen might have had a point).