Not every usage for url shorteners is for twitter. I personally don't use twitter, but when I send long links to my mother via e-mail, she has trouble unless I use a url shrinking service. I'd much prefer one that shows the domain outside of extremely space constricted environments.
If your mom has trouble opening large links, she's probably not the type to be comforted by a subdomain. If the link is from her son she's going to click it.
