Use go-template instead of jsonpath.
See https://pkg.go.dev/text/template.
kubectl get secret -l your-selector-label -o go-template='{{range .items}}{{.data.username|base64decode}} {{.data.password|base64decode}}{{"\n"}}{{end}}'
See Stack Overflow thread https://stackoverflow.com/questions/66263757/decode-base64-in-kubectl-jsonpath.