2.5D can refer to many graphical styles (isometric, emphasized use of parallax, 2D movement in 3D world, etc.), but most common to those is the emulation of 3D-looking graphics by 2D techniques only.
So, according to this definition: yes, it's possible to do 2.5D with SFML, as the latter provides the 2D tools you need. The more interesting question is, what kind of graphics style you're aiming for. Most probably you will need to invest some effort on top of SFML, as it comes with rather basic features. Most of this extra effort will be mathematics, to calculate angles and distances in your perspective of choice, and I think you will find a lot of it on the Internet.