Personally, I feel that dresses/skirts of "that" length can be just bordering on too short (if you have long arms) and most definitely too short (if you have short arms) - also taking into account leg length, a short legged girl can wear a much shorter dress than a longer legged girl....just mostly due to the amount of skin showing... you know?
Of course, I never really show any skin as it is, just what I'm comfortable with...so I'm probably a "prudish"-ly opinionated source!
The other thing that I take into account (I have friends who use me as their "modesty detector" hahahaha) is how long cut the dress is. Low-cut AND short makes it worse in my book than a higher cut neckline (no cleavage) with a short skirt length.
On the flip side, I firmly believe that mothers should build their daughters up and gently steer them in the way of modesty, if that's a big concern of the mom's. So I kindaaaa take issue with what it sounds like your mom's behavior is about the skirt length. There are ways to encourage someone to show less skin without making it some huge deal where one party feels judged and the other feels unlistened to...you know?
Maybe you could ask her about what she feels an appropriate skirt length is? Maybe her opinion isn't all that far off from your own and the real issue is that she doesn't feel listened to? Maybe you two really do have seriously differing opinions - maybe there's a compromise to be had? Or if your modesty/the type of "image you display" is in question, maybe all she needs to hear is that you're prizing your body and honoring it?
Hah, so many thoughts!
My mom is always pushing me to show more skin than I feel comfortable showing - mom opposites!