Your daughter's writing may improve as she strengthens her fine motor skills. Make sure she has lots of motivating opportunities for using her hands: fun markers, finger paints, puzzles, Play-Doh or modeling clay, and craft scissors for cutting shapes can all encourage development of the finger muscles. Writing letters and numbers or drawing in different media can also encourage children; try putting chocolate or vanilla pudding on a cookie sheet and let her write in that.
You may also want to investigate getting some occupational therapy for your daughter if the teacher thinks it is necessary. Your pediatrician can help you find a therapist in your community.
Talk with your daughter's teacher about why she is having trouble completing her work in the classroom. There may be some screening available or you may be able to work out a positive reward system to encourage work completion.