Would adding fiber glass insulation between a window and a standart plastic insulation cover help insulate the windows further
Insulating the inside of your windows will have at least one direct consequence: the effective R-value of the overall window will be increased.  That is, less heat will radiate through the window.  This will have a warming effect during the heating season (ie, when it's cold outside) and a cooling effect during the cooling season (ie, when it's warm outside).  The magnitude of this effect can only be roughly estimated so you probably won't really know how much of an improvement this is without trying it.

This will result in a secondary consequence.  The window itself (mainly the glass) will be closer to the outside temperature than the inside temperature.

During the heating season, this means it will be cold (how cold depends on your climate, of course; it shouldn't get colder than it is outside).  If you live anywhere but a dry climate, this probably means it will be below the condensation point and water from the house will condense on the glass (then run down to the sill).  Chances are, over a long enough period of time, this will result in water damage to the window and nearby wall.  If it is cold enough where you live, some of this water may freeze which could result in more immediate damage from the expansion-contraction cycle.

Conversely, you will also be constructing a miniature greenhouse.  When the sun shines on the window from the outside, it will heat up the air  space between the inside glass and the insulation.  There's no extra energy coming from the sun in this case but you're concentrating it into a smaller mass (the air in the gap).  If ambient temperatures in your climate are high enough or the window is situated to get a lot of full sun, and depending on the construction materials of the window, temperature in this air space could get high enough to damage the window.  This doesn't seem extremely likely but it might be possible given just the right circumstances.

Now, set thoughts of R-value aside.  A common problem around windows is air sealing.  That is, they make a room cold when it's cold outside not because of heat radiating through them but because of cold air blowing in through holes (or warm air blowing out).  These effects can be just as important as those due to low R-value windows.  Fiberglass insulation will do essentially nothing to improve this situation.  Air blows right through fiberglass.  The plastic sheets you put up over the window may already have dealt with this part of the problem, depending on how well you've managed to seal around the window with those.

If you really don't plan to use the window as a window, your best bet is probably to rip it out and replace it with an insulated wall.  Or perhaps with some higher-quality windows and some drapes that you can be happy with.