Its certainly frustrating that the mask covers up her horn but to be fair to Hasbro, as you mention, they dont really try to hide she is a unicorn on the packaging itself.
I do agree, I would have much preferred that the masks be made of plastic and while paper accessories have become a main stay in G4, its not as if we did not have paper/cardboard accessories even back in G1 - party hats and postcards and invites all in the Party Pack, all of which are sought after now and all of which have survived the last 30 years not too bad.
For me its about keeping the price down. If Hasbro were to start including detailed and plastic accessories in packs - rather than re moulds of ponyville which are all easily cast on multi sections in production it would bump the price of the item up. When there is little quality control involved in the line and the ponies are already so small, I would prefer they have that $5/£5 price point than $8/£8.