Okay, lemme give you a quick overview of what I think about this level.
Graphics (15%): 10/10. All the tiles, backgrounds and enemies are either SMB3 or NSMB style and fit well to each other.
Music (10%): 8.5/10. The track you hear most of the time (KRtDL Canyon.mp3) has a rather bad quality compared to the other two tracks you hear in the secret zones, but other than that, the music is well chosen and fits to the level.
Difficulty (25%): 7.5/10. Quite balanced overall. But: The unlimited mushroom supply at the beginning of Section 4 severely reduces the difficulty, the part with the Spinys and the Bullet Bills in Section 3 can be quite tricky, and the Bloopers in the water part of Section 4 are a major annoyance.
Creativity (25%): 10/10. So many awesome features, items and objects, I loved them. The Mystery Box challenge is a nice idea, so is the Orange Coin challenge.
Secrets (15%): 10/10. Well hidden and rewarding. Definitely going to keep you busy for a while.
Replayability (10%): 9/10. The level is a lot of fun, finding all three stars represents a challenge as two of them are well hidden, and also, you can always try to improve how fast you finish the level.
Overall: 9.125
Yes, I think you deserve becoming a level designer.