I understand you want to recreate the SMW map as close as possible, but I would still make more of it in Minecraft. 2D levels in a 3D game do not really fit well, so I would advice you to make the levels more 3D and thus improvise more about how the level would look like in 3D. You can use the game Super Mario 3D world as inspiration, because that is an excellent looking hybrid between 2D and 3D in her levels.
The map looks colorful and lively in SMW, because it is quite small and the grass is bright green. However, in Minecraft it looks quite barren and a little bit dull if the grass is not that vibrant. So I would place more mobs, plants, cool details and all that stuff in the map, of course inspired by the levels of SMW.
For example: some levels in SMW have a greenish background, as if you are in a forest, so you can plant or build some trees around that level in the world map. Again, you can use Super Mario 3D world as inspiration, because the world map there is also full of live and detail.
To make the grass more vibrant, you can do a few things. You can use a resource pack, you can use lime concrete or terracotta instead of grass, or you can change the biome of the SMW map into the mushroom island (or jungle, but for Mario the mushroom island is the only fitting of course) with MCEdit.
So my advice is: don't try to replicate the SMW too strict, because it will not really work out in Minecraft. Try to recreate the colorful and lively ambience of the game, in the world map and in the levels, make the levels more 3D, and thus use your imagination and Mario inspiration to let SMW really shine and sparkle in Minecraft.