I think Cecily is too stupid to be a Mary Sue
Her stupidity (or rather "simplicity") was kinda emphasized by script several times, so no guessing here.
Edit: and combination of her simplicity, her "do first think later" behavior with that distinct "nobody should die no matter of what, even the bad guys (well, Siegfried is an exception
)" attitude makes her quite enjoyable character to me
Edit 2: about Lisa being a moeblob. In a wide meaning of the term, they are all moeblobs, including Cecily and Aria (despite their age). This is determined by the series portraying them in a certain way, distinct to moeblob genre. On the other hand, this series has very strong plot (unlike "classic" moeblob series like Kannagi or K-ON or Lucky Star), and that makes Lisa, Cecily and Aria not
moeblob, but
fantasy characters.
But, we had a few non-plot, moeblob episodes in the middle, - eps 5-8. And, imo, in 5th ep. Lisa is a typical moeblob (but in eps 9-11 she isn't). Cecily is a moeblob in 7th ep. but not in 11th. Etc.
This series is half moeblob half fantasy. But, it seems, fantasy half is bigger
It's the matter of balance imo. A little more emphasis on character's attractiveness, cuteness, on her voice, her beautiful appearance, her charming manners, and you'll get yourself a nice moeblob, but making the character a part of wider sheme a little more, engaging her in a plot a little deeper returns her to "normality", turns her into your usual standard common female char again.