How to plot a two-dimentional matrix?

Is there a way, to plot a two-dimentional matrix z=Mj,j ?

Sure, if you are happy with the indices i and j to be taken as x and y values you can plot M the same as f via quickplot (= simply put M in the placeholder of the 3D-plot).

If not you would need the vectors of associated x and y values and would have to create a data structure of nested matrices like createmesh() produces.

One way to do it: M[i, j -> f(x, y) -> Surface

or Mathcad Prime

and the plot