I personally liked the transformation music mostly because of the comedic factor. However, I think the music effects certainly fit WL4's chaotic, frenzied feeling.
The thing that makes WL4's music system work is how much depth it has. It also happens when you crouch or roll and stuff so the whole thing feels dynamic.
They probably also did it because a lot of the transformations are much shorter than their WL2/3 counterparts. Fire Wario can be over in a sec if you're in a tight space, bouncy wario only does one jump.... and now that I'm thinking about it, Puffy Wario never had music to begin with. Same with bubble Wario and Icy Wario.
I still think the individual songs add a lot personality too though. I'm not sure which one I like more...
I voted for transformation music also for comedic factor. I really like some of the transformation music, they're so nice!
Though the Wario Land 4 music changes feel more crazy, works well with the mood of the game also!
For me it depends,
for example i prefer wario on fire with the music, however i prefer sick wario with the level effects.
i could go on for hours but basically if i add em up i like the music changes a smidgeon more.