Personally I don't like to see veins bulging outwards, on boobs or anywhere else.
But honestly I'm not that focused on physical appearance in prospective mates.
My own boobs do show veins, but not bulging. It's just because I'm a translucent skinned cracker who sunburns looking at postcards of sunsets. So, in certain types of lighting, you can see my veins in many places (not just chest). I think they only show up in very 'bluish' light though, like from excessively cheap tube lights. in warmer lighting they seem to disappear.
I have to admit they are a bit striking, the veins on my chest look a little like a 'lightning' pattern hehe.