Hmmm, I don't know what's going on, but the bloopers turn yellow when they are descending and when you jump on them it looks like a squished dinosaur. The two hammer bros by the vine are freaking impossible to get by without taking a hit. That room with the red switches is terrible (section 8). Needs to be a much longer switch. Even looking at the editor I can't figure out the path to the red star.
E: Wait, I think I figured it out. Man, this level is freaking hostile. That room in section 2 is not fun. Why, on top of every other obstacle, put a poison mushroom behind the hammer bro? Those things are unstoppable, and they way it's positioned, even if you stomp the hammer bro you might hit the damn poison mushroom. And I still say the koopa on the moving platform is too erratic/buggy. And then you pop out go knows where and you have to find a tricky P switch, make some tricky jumps (the one with the wall blocking you and the off-center doughnut block may be impossible when you are big. You have to move the doughnut block over under the wall). as you rush to the pipe that just gives you access to the half of the stage where the star is hidden. All without a midpoint, because there's only one midpoint in SMBX. I don't think that's reasonable for someone to figure out on the fly, and it's a long level for trial and error. Maybe I'm just a wuss (which is possible), but this level feels like it's crossing the "rom hacky" line in challenge.
Try to not put lava on the exact same plain as ground. It has a weird hitbox and will kill you way earlier than you think it should. Right before the skull raft, raise that ground (AND the ceiling above it!) a tile above the lava.