In English that sounds a bit weird to me because to me someone is either bald or not. “very bald” seems a bit like “very dead” or “very male”… Or is this different in Welsh and someone with a little hair (or just a bald patch) is considered moel iawn?
There are various degrees of hair loss, or baldness, depending on age and genetics. In English, in the UK, the phrase very bald would indicate a more severe hair loss than an phrase like 'quite bald'. The Welsh expression reflects this.
There are also various idioms referring to degrees of baldness.