In my opinion no she isn't a Mary Sue. Mary Sue means an idealized character, often but not necessarily an author insert (representation of author). "Wish-fulfillment" implication: the "Mary Sue" is judged as a poorly developed character, too perfect and lacking in realism to be interesting. I don't think that Twilight is a poorly developed character and I think she has obvious flaws which make her imperfect. Twilight has grown up a lot through the show she's one of the most developed characters and sure she may be princess now, but that doesn't mean she is perfect. Her flaws include, but are not limited to she is stubborn, she over thinks things, she's been a bit sheltered so she doesn't quite understand some basic things (this is more the first season), and she's a bit bossy. Mary Sue would be a static character, a character that doesn't go through change and Twilight is the complete opposite. The show revolves around her and her friends and the lessons they learn together. The show is about growing up and becoming better people. So again no I don't think so.

She's not an OC so she's not a Mary Sue. Mary Sues are usually OCS

Sometimes she can look perfect but she's no where near perfect. She's often has the desire to know everything and at lest knows what's going on that she gets caught up in it. And just like Courtney has a desire to be perfect but never filling that role. And she's also over protective about most thin things like when you saw her picking up cupcakes in lesson zero she was over protective about every one having the equal icing on cupcakes that she got everyone little amount. What I love about twilight is that unlike her stereo-type her character doesn't relay on her being smart to see her flaws and weakness. The writers also don't force it into our face that she's smart. Not counting Wedding episode and weather fall friends but besides that she never really looked like a marry-sue.In lesson zero is where you can see her flaws on display the most. And also unlike her stereo-type she didn't have trouble making friends she had trouble wanting friends. Which is very like me I hate having friends and having and feelings at all and a marry-sue is hard to relate to which is why that stereo-type is so hated and Twilight is one of the most relatable ponies in the show. And she went through a lot of development and her flaws wasn't by the showing trying to make a perfect character. Even through I can see why people debut that she's a marry-sue more than a smart type. But even through it is my lest favorite stereo-type to me she fit the smart stereo-type more. So no she's not a marry-sue at all.

No, because she's not perfect. She has many character flaws just like everypony else. She's had to learn lessons and the importance of friendship, and she's learned things from the others. 'Lesson Zero' is a perfect example of her flaws. Just because she's smart and an alicorn now doesn't make her a Mary Sue, that would mean that Celestia, Luna and Cadance are Mary Sues just because they're alicorns too. She's had to grow up and find out who she truly is.

The definition of Mary Sue is a character that is perfect in every way, shape or form. Obviously, Twilight has flaws. Need I say more?

Wow! Who ever said yes is impossible to tell his/her reason without causing a debut or fight.

No, because she has her flaws, like everyone else. Celestia has flaws. Luna's flaw at least to me was that she was jealous of her sister getting the spotlight, and turned to destructive behavior-and what do you know, Sweetie Belle became the same way in "For Whom The Sweetie Belle Toils". She was jealous of her sister Rarity, and wanted to ruin her dress to get revenge. Then Luna comes in and shows her what a bad idea that is, because she used to be jealous of her sister when she was younger, and wanted to be the only princess.