The jMonkeyEngine SDK bundles many tools, but a mesh editor is not among them. This is because there are countless flavors of 3D modeling tools: Blender, Maya, 3DS Max, Trimble SketchUp, MilkShape3D, ArtOfIllusion, and many others. The choice depends on your operating system, your work style, your level of expertise, and finally, your wallet.
The jMonkeyEngine does not restrict you to use one particular mesh editor. You can keep using the tools you are familiar with. The only requirement is: your 3D modeling software must be able to export files in a format that jMonkeyEngine can load. At the time of writing, the jMonkeyEngine supports Wavefront, Ogre XML/Ogre DotScene, and Blender binary formats.