I, for one, have taken that test for many times for several different characters and a good amount of the time the results are the same, or only changed slightly. Don't rely too much on the results for adequate feedback as to whether or not Scribble is a Mary Sue.

Now I'm going to say this right now, being a Mary Sue in fiction is by no means a bad thing. Many people consider the character Nyx from Past Sins to be a Mary Sue, and in fact she is. But the story and character is written well enough that it's only noticeable if you look at it critically, and the same goes for Scribble. There is an express difference between being overpowered and being a Mary Sue, and it's quite easy to be one without the other, but most of the time you'd find most poorly made characters are both.

Scribble is by no means a poorly made character. You took time and consideration into making her, and it shows with how deep her personality is. In all honesty you can owe a good portion of that to writing in first person. Scribble is a very smart pony, but she isn't a genius that understands every subject like it were her right. You've balanced her current and future strengths to almost complete equilibrium, and that shows just how much you care about her.

I remember what you said about Scribble once before.

I love her, but I love her enough to let her fail

That mentality alone makes Scribble not a Mary Sue in my eyes.