Can having pectus excavatum cause breasts to become deformed?

Not usually. Generally pectins excavatum doesn't affect breast development. Poland syndrome is one situation when pectus and abnormal breast development occur together. Usually pectus excavatum will make the breasts seem less prominent.
Pectus Excavatum. Female patients with significant pectus excavatum will frequently have associated breast asymmetry and rotation.
Yes. Depending on the depth of your pectus your breasts will project differently. The breasts sit on the pect muscles under the skin of the chest. In pectus ecavatum, the breast bone is depressed and malformed so the chest is depressed and the breasts can appear assymetric or rotated inward or even smaller.