NO, only 19.4% of American billionaires are Jewish in 2016
Less than 33% of American billionaires were Jewish in 2014 (this is an overestimated upper bound, read below for full details).
According to Forbes there were 540 American billionaires in 2016:
America boasts 540 billionaires, more than any other country on the planet and more than all of Europe combined.
Forbes Israel compiled a list of all Jewish1 billionaires in 2016, including their nationality, on the list there are 106 people with US citizenship starting with Larry Ellison, 5th richest man on earth and ending with Charles Zegar, 1342th richest man on earth.
In total there are 106 out of 540 which are 19.4%.
Also, according to the same list and the accompanying article Jewish billionaires comprised 11% of the world USD billionaires.
In 2014, according to Forbes there were 492 US billionaires, by taking the list of 2014 billionaires and comparing to the 2016 list2 I got a list of people who were billionaires in 2014 but not in 20163 there are at most 59 people who lost their billionaire status between 2014 and 2016, to get the highest possible number of Jewish US billionaires we would have to assume that all of the people who stoped being billionaires were Jews, and that all the Jewish billionaires in 2016 were also billionaires in 2014. This gives us a theoretical limit of 165 Jewish billionaires out of 492 which is 33%.
I can't tell for sure how many Jewish US billionairs there were in 2014, but it was at most 33%.
1 - The list is comprised of practicing Jews like Lev Leviev, non-practicing Jews like Mark Zuckerberg and people with Jewish ancestry but who are not considered Jews by the Jewish religion like Nicolas Berggruen.
2 - I copy-pasted the lists, used a Python script to compare the lists, and ran it on repl.it. I'll upload it once I figure out the best way to do so.
3 - There maybe some people who are in both lists but the script missed due to different spelling. The people who lost billionaire status includes people whose worth was reduced as well as people who died in that time.