It is personal preference, and it has nothing to do with performance. So if you love her face, continue to love it. She's your girl. :)
An upright shoulder limits the lift and length of a stride, makes it harder for a horse to lift up for jumps, can make the horse very rough to ride (although that is debatable) and can occasionally cause chronic pain because it puts a lot of pressure on the front legs/knees. It is not a horrible conformation fault for pain (like back at the knee, or long pasterns), but it is bad for performance and overall balance of the horse. Compounded by the fact that she is downhill already, you're going to have your work cut out for you to properly collect her--but it will be the only way to keep her sound for a long time. :) It's doable, it's just not as easy as it is with a better conformed horse. If you plan to jump her, make sure you do a lot of dressage to rock her back onto the haunches so it will spare her front legs, and always put boots on her legs to help!