As for the other areas mentioned, like Link's Awakening and Holodrum/Labrynna, those make perfectly sense in my opinion because these places had characters that had some originality to began with, they were truly denizens of their world. Whereas Termina was lazily crafted.
I think you might be forgetting that characters like Guru-Guru, Tingle and the Happy Mask Salesman show up as inhabitants of Holodrum and Labrynna. They are as guilty of 'laziness' as Majora's Mask is in this regard.
The engine, that's another matter. Yes, they used the same engine from OoT for Majora's Mask, which is why so many of the characters looked the same. That you can pass off as laziness.
I wouldn't say that was laziness. Using the same engine was an efficiency thing. By using the same resources, they could create a new game in a very short time. I suppose the idea that "they aren't the same because it's a parallel dimension"
could be seen as lazy, but I don't agree when you consider the lengths they went to to make sure we knew they weren't the same characters.
Take Anju for instance. They reused the Cuccoo Lady's model, true, but Anju has so much more character, depth and life to her. Creating the entire Anju-Kafei story was far from lazy. Romani and Cremia are another example. They reused both of Malon's models but rather than just have a pretty girl who liked horses, they created a family unit that faces hardships in the form of violent business rivals and even invaders. Considering how Cremia lets Romani try some Chateau Romani milk in the final hours of the Third Day, even though it is restricted for adults only, you can see that Nintendo created some fairly deep characters that brim with emotion.
More than that, most of these kinds of character interactions aren't brought to to the fore, meaning you have to go explore everywhere at every specific time to truly see the whole scope of the game. Creating a world that deep, that connected and (forgive me if I sound like a twit now) that
real came about because they had reused the models but needed to identify the new characters strongly so we wouldn't confuse them for their counter-parts. Reusing the models forced Nintendo to work harder and the result was a game that has an immeasurable amount more depth, charm, wit and genuine emotion than Ocarina of Time ever did.
I just don't see anything about Majora's Mask as being "lazy".