Your mapfile is mostly OK. I used the states.shp shapefile from the GeoServer demo data in my test and converted it into GeoJSON with ogr2ogr
ogr2ogr -f geojson geoJson.json states.shp
I checked with ogrinfo that my geoJson.json file contains a layer named "states" and used that in my mapfile. Other edits that I did are
Because you used the relative path I created also a subdirectory "shp" under the directory where the mapfile is located and plased the GeoJSON file there. Myself I prefer to use absolute paths but relative paths are OK as well.
#EXTENT -120 34 -119 35 # Geographic
EXTENT -180 -90 180 90 # Geographic
SIZE 800 400
OWS_TITLE "MetGeoJSON Polygon"
COLOR 200 255 0
OUTLINECOLOR 120 120 120
END # Style
END # Class
I used this query on my browser for testing
The result is this
I suppose that your GeoJSON file is not in the correct directory that is relational to the mapfile location, of the layer of the GeoJSON is not "OGRGeoJSON".
Your mapfile is very minimal but it does work even without CLASS and STYLE. You just won't see anything on the map because nothing is rendered. For next tests I suggest to use the mapfile from the WMS Server manual page as a template. It is still short but contains useful examples about how to configure projections and basic styles and all that is needed for setting up a WMS service.