Rhiow thinks that Katara takes too much onto her shoulders, takes things a little too personally, and allows herself to be wounded. But she recognizes that all of these things are the mark of someone who hasn't quite fully matured, so she views them with a kind of amused forbearance.

She does recognize Katara's considerable skill with her own Mastery of waterbending, she just needs to grow a little farther into an adult - then she'll gain the wisdom to go with it.