Maybe, but I still don't think she earned it. She isn't even part of the royal family, that if there is one, Cadence isn't even too. The Royal Family of Equestria is only Celestia and Luna, I think, maybe Blueblood, but him, not so sure. He isn't even an alicorn. What I think a pony needs to show to be worthy of the title Princess or Prince is good leadership, being able to stay calm in a crisis, and need to be caring of their subjects. Twilight Sparkle might be a good leader, but can she rule an entire kingdom? Cadence cares about her subjects, but she didn't show any leadership. These are just my opinion, they could be right, and they could be wrong. Both Celestia and Luna are good leaders, capable for being calm in a crisis, and are very caring about their subjects. They both deserve the title.
Making them princesses just to sell more expensive toys? That is a stupid reason. So, but it is. If I was the head of Hasbro, Cadence will be just either an unicorn or a pegasus, and Twilight Sparkle remains an unicorn forever. She can be like the great unicorn sage of the era, but not an alicorn. She is smarter than Starswirl the Bearder, I guess, so she could be the Starswirl the Bearded of the modern era. That might works well. Makes more sense to me.