No.
There are no functions for creating only thumbnails.
It also is not possible to create a new transcoding job without actually transcoding anything. The input parameters require, at minimum, the name of an input video. The output parameters require, at minimum, the name of the output file and a preset ID. Parameters are checked prior to starting the job, and there are no options which would prevent the job from executing while creating a thumbnail.
You can read about all of the available functions here:
http://docs.aws.amazon.com/elastictranscoder/latest/developerguide/api-reference.html
Give ffmpeg a look. It can be a little bit of a hassle to install, but it can create thumbnails from videos.