Although I don't really see the steampunk elements, I love this work. It's very beautiful and delicate, and the different colour versions work very well together.
You might be interested to know that I've included your piece in an article on videogamegeek.com about great fan-made posters. Of course, the article credits the work and links back to this page. Here is the link: videogamegeek.com/blogpost/233…
The art deco and Victorian style of this, the suggestion of star charts and theatrical elements, and the monochrome versions evoking old-fashioned printing methods make this evoke the steam era in a more genuine way then throwing gears all over everything, which is what many people do to make something 'steampunk'! Awesome, and don't let anyone tell ya otherwise!
This is really a beautifully rendered piece. You can really see the 2 sources of your inspiration, the Mario world/characters and the Space Mountain poster you linked, coming together into one cohesive idea that really stands strong on its own. Really awesome stuff.
The layout of the poser also has an antiqued Victorian layout to it, which is mostly what I was going for. Rosalina's dress is also different, and I gave her and old telescope. I drew the shroomship as a blimp, but I didn't fit anywhere in the poster.