To clarify what I meant about the episode:
I'm speaking from the perspective of a fellow creator. It is far more important to ensure the product suits the client's needs than it is to make it look beautiful. I draw pictures. If someone asks me for a specific idea, and I draw something else, it didn't matter how gorgeous it was. I still failed to meet their request.
Rarity is an artist. She has a talent for creating beautiful expressions using fabric. She's highly skilled with form and function. As many episodes have shown us, she's extremely detail oriented when it comes to her creations. He's also a professional, and would understand what it means to work for the customer.
My problem with this episode is that it breaks her character by ignoring this in order to make the plot work. The entire episode hinges on her completely ignoring the function of her client's order. The stallion needed a traveling puppet theater. This requires three things; that it be sturdy, there be enough room to put on a show, and that it can safely travel. Any novice can design this. The function is key, the form is secondary.
Rarity is no novice. She's a creative expert. Yes, her focus is clothing and fashion, but that talent does bleed into other areas. Also, I find it difficult to believe she wouldn't seek help with understanding the construction if she didn't know how to handle this. She has a close friend who owns a book on almost anything you can imagine, and I'm sure there's a carpenter in town. There's no excuse for her not being able to handle this competently.
Basically, this episode resorted to the lobotomy technique. It forces a character to do something uncharacteristically stupid in order to allow the plot to follow. This is exactly the same reason A Canterlot Wedding was so terrible. The mane six were required to forget one of the earliest episodes in the season so the plot could work, as well as ignore key details inside the episode itself (like Twilight being VERY familiar with Cadance).
Ignoring the terrible intro, the rest of the episode is just fine and quite enjoyable. I thought Spike was a bit dim at times, but he was never so stupid that it seemed to be out of character. He is still a dumb kid, after all. I did love Twilight being all pissy at the end. It seemed very appropriate.
This is what I meant by lazy writing. The writers of this show rely on lobotomy far too often. I -really- wish they would stop because it makes the characters seem like they are, in fact, learning nothing and never change. This is contrary to the entire premise of the show's running theme, which is, "Today I learned about..."