Hey there, I thought you might like my feedback on this =)
I am not one who is into ABDL, but my boyfriend is. In a sense, I am in your girlfriends' shoes.
From my point of view, I see my boyfriend as the expert on the subject and I aim to please him in this new adventure we're sharing. While I haven't changed him as of yet, I would not think it an insult or something negative if he gave me feedback on my diaper-changing skills. In fact, I hope he'd give me feedback.
Why?
Because my first and foremost goal is to make him happy, and if I can get better at that, why would it be negative? Also, learning implies making "mistakes" and learning from them. Your girlfriend is probably no expert on adult diapering, so learning new things is not a negative aspect.
I hope this helps you! Of course, communication is key and the tone you will take will affect the conversation... But from what I could read, your last wish is to upset her- and so I think that will also show through your words.
Good luck!