Actually "V_MPEG4/ISO/AVC" is the correct Matroska ID for H.264 / AVC video streams in a Matroska container so nothing wrong with that.
You could easily change this to some other value you like more. One way to do so: group your collection by MediaType than select all the movies listed under V_MPEG4/ISO/AVC and use "Multiple Changes at once > Change MediaType > ...".
If you just need an icon set you would have to add V_MPEG4/ISO/AVC to your Media Types list (Tools > Manage Lists > Edit Media Types (Icons)).