Been playing this for a few hours today. I agree with much of what has been said already.
I have found the Map in the Amazing Mirror (AM) to be pretty obtuse. Part of the problem seems to be that the map lacks the detail neccessary to work out spacially where Kirby is within a given box. Akin to Super Metroid, AM has a map system which is comprised of connected 'boxes'. Also like Super Metroid, it can be difficult in AM to determine where you are within a given box. You usually have a rough idea of which direction you need to go in order to reach the next box, but it's not always apparent which door will lead in which direction. It can be a little frustrating.
I have a theory for why some people, myself included, might find the Map somewhat frustrating. For the purposes of comparison I think it's worthwhile looking at Super Metroid, which is perhaps my favourite game.
Why, you may ask, would a guy whose favourite game is Super Metroid find the map in AM to be frustrating given that they're so similar? I believe it is because Super Metroid is a better game.
Now, while that may seem like quite a caustic statement, for me the overall quality of the game has repercussions for how appropriate the map system is. With Super Metroid, the game is fun enough for me to want to explore. The level design, combat and general sense of atmosphere in Super Metroid suck me in to such an extent that I feel compelled to explore the world. With a game like Super Metroid, therefore, all I really need is a basic map which highlights where I've been and where I've yet to explore.
With AM, however, I don't find the level design, platforming or enemies to be all that engaging. As a result, I'm not looking for a map which is as abstracted as that seen in Super Metroid. What I want from AM's map is something which shows where exactly I need to go to move on. It might be a strong indictment of the overall game that I simply don't want to have to explore in AM, something which I love to do in other (I would argue more well made) games.
In my opinion, therefore, the problems some people have been having with the map are more reflective of their experience with game overall, than they are with the design of the map itself.
I could be wrong though.