As a Roblox developer, it is currently impossible to create meshes with transparent features without changing the transparency of the WHOLE mesh to 0.1, EXAMPLE: 
As you can see even though the texture is transparent, the mesh is not. After all there is a way to fix this, but it comes with a few set backs, to fix this issue you need to change the mesh part transparency to 0.1 here is a list of the setbacks in doing so.
The transparency tends to mess with chat bubbles making the mesh show up on top of the chat bubbles, this is especially annoying for creating plumes on hats (Like the one pictured above). EXAMPLE:
As you can see the plume is glitching in front of the speech bubble due to it’s decal and transparency.
The other meshes who also have a transparency tend to have overlapping issues, EXAMPLE:
EXAMPLE 2:
As you can see the person who is in front is wearing the green plume, yet the transparent plume from the other person is still somehow showing up through the green plume, and in the 2nd image you can see a mesh part showing through another transparent mesh part (the tree).
And finally is an odd shadowing error (I assume) EXAMPLE:
You can see the shadows inside the plumes and near the center, giving it an unnatural glitch appearance.
If Roblox is able to address this issue it will not only make creating hats easier, it will allow many games to create a more immersible experience and allow chat bubbles to be read much easier. This will also allow the creation and use of mesh trees to be easier and more widely used if this issue is addressed. A possible way to do this is to somehow implement an alpha system to make the mesh transparent with the texture, you can find this feature in many 3d Modeling programs, such as Cinema 4d R15.