Mulgrew would have been able to pull off bald just fine, only problem being as soon as you have a bald woman it's some kind of Big Deal.
She has the face for it. Shatner just doesn't, he would not have looked good bald even in his prime, or not as good. When he got older he would have ended up looking like a Sopranos extra.
Why would a bald Janeway be a big deal? We already had a bald woman in TMP!