Creating a fake letter doesn't make sense as Kasich did vote for this ban-
https://www.govtrack.us/congress/votes/103-1994/h156 ), However, this has been floating all over facebook for about a week and the legitimacy of it is in question. Point one, that looks nothing like Bill's signature (too clean and rounded, Bill had bad handwriting). Point two, the formatting doesn't look like other letter's from Clinton when he was President (example, location of signature and salutation lines, etc. the other thing is the White House seal is not on the letter but that may be due to cropping.
Example of an official letter from Clinton from just 5 months before this.
And another:
And another: