It is certainly not smooth. In fact, Shishikura showed in '94 that the boundary is itself a fractal with Hausdorff dimension $2$ which informally, is very jagged, and is the same dimension as the Mandelbrot set itself.
See:
> Shishikura, M. "The Boundary of the Mandelbrot Set has Hausdorff Dimension Two." _Astérisque_ , No. 222, 7, 389-405, 1994.