I think often these kind of things are a diversion to draw our eyes away from other 'things' in the image. Yes ok may look like a mouse to the general population, but there are other more obviously anomalous pipes and blurred-out areas in this linked image. So, although it does look like a mouse for the readers of the Daily Express or Daily Mail or whatever paper posted it originally, it is, I feel, just a diversion to get their readership up and pull the plonkers of their tabloid readers. Have a closer look at the linked image from mars.nasa.gov to see what I mean and look at nearer areas to the camera.