Every element of a cell array is allowed to be a different type, include structures, objects, multidimensional arrays, strings -- and other cell arrays. Some of those cannot be displayed as text, and some of them are a nuisance to display as text (e.g., 5 dimensional arrays of structures of multidimensional arrays of numeric values.) It is therefore not generally possible to display the contents of a cell array in a msgbox.
Displaying the contents might be practical if the form of the contents is known and fixed.
If the contents are strings you might wish to look at strjoin()
