It's hard to tell much at this age and from pictures where the chi is not stacked up, but an underbite is a fault in the show ring, you could show a chi with an underbite but would get a serious fault for it. I have seen a slight underbite change to a level bite before if it's slight, their bite can also go from good to bad as they grow up which is why a chi can never be named as a show quality dog before it is six months old, so much can change!
I would wait to see how she develops and try taking her back to your breeder to be evaluated. If conformation showing doesn't work out for your mom there are alot of other fun things she can do show wise, agility, obedience, etc.
She's very cute either way!
~*~Gypsy, Deedlit, Cosette, and Isabeau~*~