If you collect every coin in the game, you unlock a purple Yoshi.
A prank created to waste your time.
In some of the jungle levels, there is a voice in the background music that says "Mutha f---a, hump 'em!".
It's actually just random jibberish that's a part of the music; it doesn't even form real words in any language, much less swear words and/or other adult content.
Also, unless your TV sound is either broken or turned down very low, the aforementioned gibberish doesn't even sound all that similar to said phrase.
There is a secret area of Jelly Pipe in which you can 'swim' in/under the jelly; this leads to a secret exit which will let you skip to any level in the game, including directly to Baby Bower's castle.
This may have started as a slight twist or corruption on the idea of the secret warp zones in the original Super Mario Bros.
Tubba Blubba from Paper Mario originally appeared in this game as a boss.
Not true; however, there is a boss named Don Bongo at the end of the level 'Frustration', who is a tubby, lizard-like creature, and who may have been the source of this rumor.
The yoshis in the game are all intended to have specific genders: the 'darker'-colored yoshis (for example, dark blue) are all male, and the 'lighter'-colored yoshis (for example light blue) are all female, and the gender yoshi you choose for a level somehow can affect the difficulty of completing said level.
Nothing in the game whatsoever states or supports the idea of specific-colored yoshis having specific genders. On a side note, it could be possible, though, for one to want assume that the pink yoshi might be a girl, but even so, again, nothing in the game really shows this to to be true in any way. Even it if were, however, it still wouldn't matter, because there's no obvious or sensical way in which a yoshi's gender could effect level completion.
If you look closely, there is a UPS logo in the background of one of the levels.
No, but in at least one level, the 'sky' has the word 'UP' and an arrow, hinting at the idea that the background is made of a cardboard box with a 'This end up' symbol on it.
The yoshis' singing on the title screen is supposed to form some kind of real words, some known ones including "Wa-waffle, waffle!", "Wa-water, water!", "Wa-walker, walker!", and even in one strange and isolated example (which may have even appeared in an issue of Nintendo Power in a letter at some point, if memory serves correctly), "Oh, at the airport!"
Again, like in the other example, this is only gibberish, and not meant to be anything but babyish babbling/singing.