Yeah maybe add a few textured tiles sporadically throughout the solid blue part. That would make it more aesthetically pleasing without compromising what the blueness brings to the feel of the room.
You know, those "steps" on the ceiling really look odd to me. I get the feeling that this isn't really supposed to be too natural looking, but I don't get why there would be that formation on the ceiling. This is just my opinion, so I may be the only one bothered by this.
Aside from that minor quirk, the room looks pretty good, but maybe it could use more details. I don't know, I'm not that good with designs.
Vary the tiles that are outside the 'trim'. You've only used the plain blue tile, but there are other filler tiles. Failing that, there's the false layering thing that I feel like I'm mentioning every other day at the moment.
Also, when you have a smaller room like this, there's no need to use the included background, as it won't scroll, so you should set it to a custom layer 2, which will give you more freedom to create more depth and detail.
Stalagmites, stalgtites, BG, lava, varied terrain (like open areas in the floor), FX1, false layering, behind the outer tiles (the ones you touch) add detail as there is a whole array of tiles for thy exact purpose. I hate to bring this up once again but check out redesign, it has an excellent quality of detail an you could learn a lot from it, just be sure not to exactly copy it.
Ok, I'm almost done Apalite so I'll retouch any rooms needed. I only have to put in ridley's fight and a couple more rooms then a new place.
Also I'm not using lava with Icy cores because it don't work right. I use water. Also do you mean the BG tiles? like the tiles the are black for Crateria?