Photoshop 3d extrusion renders pixelated paths
Based on the image I would say that you need to render it. The preview in 3D is quite crude.
There are two things to consider when creating 3D in Photoshop CS 6 Extended.
First, as Alan stated, (for best results) you'll need to Render your scene. Think of this as the final step before saving for web/or use. I like to do renderings intermittently to see how things look like lights, shadows, reflections, etc...
TIP: You can use the Marquee/Select tool to select an area of the 3D Layer in Photoshop and just render out that sectionâ€”much faster then rendering out the whole scene.
Second, document resolution plays an important part of creating 3D Extrusions. That is to say, the larger the document/shape the better the resolution of the 3D object being created.
TIP: Start with a document size that is 2-4-10x larger than your "final" size you'll be using. Create your outline, vector, shape, text, etc... and then create a new 3D Extrusion. Once you have your shape, use Image Size and make the document the size you want to use. This will make the canvas smaller, but will keep the high-resolution 3D mesh/object you created.
Note: I would also suggest that you consider adding a small bevel to the front of the extrusion, as in the real wold, nothing has infinitely sharp corners (which is what most 3D apps created, even Photoshop). You can add a contour to the bevel if you'd like it to be a smoother transition from the extrusion side to the font face. (You can do it for the back face as well.) Adding the bevel to the edge of the text/shape/3D Extrusion will also allow the bevel edge to pick up any highlights from light sources in your 3D scene.
Also, if you're new to 3D in Photoshop check out the Photoshop.com 3D page. http://www.photoshop.com/products/photoshop/3d
It has materials, IBLs, Stages and other content you can download as well as a link to Photoshop Dimensions, the Magazine of 3D in Photoshopâ€”the fist issue is free and provides an overview of 3D in Photoshop CS 6 Extended.
I hope this helps. Cheers!