Did you vet tell you anything more about hematomas and how they are normally treated?
Basically, the term hematoma is used when you have any kind of abnormal space underneath the skin that fills with blood. Hematomas are generally caused by injury which breaks blood vessels under the skin and causes a pocket to form and fill with the blood. In cats and dogs, it's usually due to shaking their heads or scratching their ears due to allergies or illness, but it can also be due to playing with other dogs, and the like.
There are a couple of different ways a vet can treat a hematoma. The first is aspiration, which means draining the hematoma. Since that doesn't close the "space" created by it, it can fill up again, so this isn't a very good fix. The second is surgery, such as what your vet has probably planned. Here, they drain the ear and then tack the two sides together to hold the "gap" caused by the hematoma closed until it heals up. And the last option is o open the ear up and place a drain in it and let it drain and heal.
Surgery is the easiest way to fix it and usually has the best results, although if the hematoma is at the tip of the dog's ear, there is no absolute guarantee that the ear will look just like before or that it will remain standing. It is not uncommon for ears to flop over at the top if that's where the hematoma was located.
Hematomas in the bottom part of the ear sometimes can be left alone as the blood does eventually absorb, however, the ear won't go back to normal, it'll look somewhat bumpy. (They call this "cauliflower ear".)
I would recommend you go with your vet's recommendation about fixing the ear, especially since it is the top / tip of the ear where the hematoma is located. That'll probably be the best chance of keeping the ear a normal shape.