In an article at Huffington Post, Forte admits that his mother was horrified by it. As she should be. I'm sure because she raised him better than that. If you're going to debase yourself and humiliate your mother you might as well go with something respectable like a carrot or a nice, firm zucchini squash.
If you're not utterly convinced that Will Forte will do anything for money you can see the evidence yourself here: Celery Scene from MacGruber
Adding insult to injury, Forte goes on to actually eat the celery.
(Please do not watch if you have some kind of emotional infirmity. Or a pacemaker. Or are at work.)