Applescript to set Track Comment to Rating
First thing is that the rating value is just stored as "rating". But iTunes stores the star rating as a value between 0 and 100, so you need to convert from that value to the number of stars.

There might be a smarter way to do this but this code seems to work.

tell application "iTunes"
    set theTrack to (item 1 of (get selection))
    set theRating to rating of theTrack
    if theRating = 100 then
        set comment of theTrack to "5 Star"
    else if theRating ≥ 80 then
        set comment of theTrack to "4 Star"
    else if theRating ≥ 60 then
        set comment of theTrack to "3 Star"
    else if theRating ≥ 40 then
        set comment of theTrack to "2 Star"
    else if theRating ≥ 20 then
        set comment of theTrack to "1 Star"
    else if theRating = 0 then
        set comment of theTrack to "0 Star"
    end if
end tell