I'm assuming that you've pulled this from the example in the documentation for the MatrixFieldOfValues ResourceFunction. So I don't know whether you really want a parametric plot or if any visualization type will do. I'll show one way to do it with FilledCurve.

Graphics[
 FilledCurve[
  Line[Table[
    ReIm[ResourceFunction["MatrixFieldOfValues"][mat, t]], {t, 0., 
     2. Pi, .1}]]]]

Couple of things to note:

• The general strategy I used was to create data representing 2D coordinates, wrap it in Line (since Mathematica has useful functions that operate on Line structures), wrap that in FilledCurve (to represent the idea that we're wanting to visualize the filled region bounded by our line), and the trigger the display with Graphics (without which you just have the structure that represents the semantics of a filled region bounded by a set of line segments).
• The "."s (periods) used in the range for t specified by the Table force the ResourceFunction expressions to evaluate.
• The example you provided, by the way used 2[Pi] instead of 2 Pi, and this generated an error in the ParametricPlot.
• You can change the "resolution" by changing how many "samples" you use in your Table. In the range specifier, {t, 0., 2. Pi, .1}, the .1 is the step size. Make it smaller to get more points and a smoother curve, bigger to get a chunkier curve.
• You can look at the documentation for FilledCurve and Graphics and Line, and use the info to change the appearance of the final output (colors, textures, edges, etc).

A little trickery is needed if you want to make a filled region out of the field of values function:

With[{mat1 = ExampleData[{"Matrix", "WEST0067"}]},
     Show[ParametricPlot[ReIm[ResourceFunction["MatrixFieldOfValues"][mat1, t]],
                         {t, 0, 2 Pi}, Axes -> False, Frame -> True, 
                         PlotStyle -> Directive[EdgeForm[Directive[AbsoluteThickness[4],
                                                                   ColorData[97, 1]]], 
                                                FaceForm[Opacity[0.4, ColorData[97, 1]]]]] /.
               Line -> Polygon, 
           ComplexListPlot[Eigenvalues[N[mat1]], PlotMarkers -> Automatic, PlotStyle -> Red]]]

where I use PlotStyle to insert any useful directives, and then replace all instances of Line[] with Polygon[] afterwards.