I do not have enough context to determine if surprising someone with a puppy would be appropriate or not. Some considerations around gifting a pet as a surprise:

Having a pet is a significant financial, time, and attention commitment. The recipient needs to be committed to and equipped to do that duty. If you catch someone off guard, they might not have enough time to decide if this is the ideal moment.

It’s best for pets like dogs to be selected together so you can find one with the right temperament, activity level, etc to match your lifestyle.

Some people may have allergies, landlord or housing restrictions, or other barriers to pet ownership you may not be aware of. A surprise could put them in a difficult position.

My suggestion would be to have an honest discussion with her about whether getting a puppy is something she wants at this point in her life. After agreeing to proceed with the puppy together, a romantic reveal could still be planned if she is excited about obtaining one soon. Making the process collaborative rather than completely unexpected should be the main goal.