Clothes make the man, right? We don't see Alanna in a dress, like, for the whole book, so there's something to that in her case.
When Alanna first arrives at the palace and is totally freaked out that someone might discover her identity and send her home in shame, one of the things that calms her down is her new page's uniform: "She had never looked so fine…There was one thing to be said for such a bright red and brighter gold: the royal uniform gave her the courage to unbolt the door and step into the hall. She couldn't have done it in her battered old clothes" (2.34-35). To recap: clothing = confidence-booster. Yep, we hear that.
At the same time, simply dressing nicely doesn't necessarily make you a nice person. Take Duke Roger: snappy dresser, but he's almost certainly plotting to kill his cousin and assume the throne. When he interrogates, er, politely questions Alanna about having the Gift, he "glimmered in a many-colored tunic and red-purple hose" (5.77). That is quite a fashion statement, sir. Too bad you don't have any morals that match that outfit. Oh snap!
The take-home lesson is this: people can use clothing to send a message about themselves, but the truthiness of that message depends on the honesty of the person sending it. Alanna is super earnest and honest with how she represents herself (with the one exception of hiding her gender), so we can generally look to her clothing for some indication of who she is. Duke Roger, on the other hand, is a mysterious dude who is most likely intentionally trying to convey that he is likeable and wonderful—in order to hide the fact that he might not be those things. Tricky.