I am afraid that there is no way to remove the OS default icon from text files. You can only use a view that doesn't show icons at all.
Also, I don't think MC has a built-in way to mass link cover art with your txt files.
Here is a way to do it outside MC, but you need to find an external tool that can batch process the file modifcations:
1) Create a list of the txt files (a playlist, a view with a restrictive rule or just load the files to Playing Now) and export the list as a simple .m3u playlist.
Here is an example of an exported .m3u list:
H:\Rip\Dido - Life For Rent\a sample text file.txt
H:\Rip\Doina Klezmer - Nomada\another sample text file.txt
2) Change the .m3u file to this format:
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<MPL Version="2.0" Title="text folder">
<Item>
<Field Name="Filename">H:\Rip\Dido - Life For Rent\a sample text file.txt</Field>
<Field Name="Image File">folder.jpg</Field>
</Item>
<Item>
<Field Name="Filename">H:\Rip\Doina Klezmer - Nomada\another sample text file.txt</Field>
<Field Name="Image File">folder.jpg</Field>
</Item>
</MPL>
3) Save the resulting file as an mpl playlist (.mpl) and import it by using the "File > Import..." command. It will add cover art links to the listed txt files (assuming each folder contains a folder.jpg file).