God willing!

“God willing?”

"God willing" usually follows something else that you hope and/or plan for, such as "tomorrow it will be sunny, God willing."

So you should probably punctuate it, " . . . God willing."

It does not really need punctuation unless you have further meaning in mind.

It could be more meaningful to quote the entire line from the Lord's Prayer, "Thy will be done on Earth as it is in Heaven."

. . . but leaving it without punctuation could open it up to contemplation more.

Here is something you can print up for your mom for it is nothing but pure and simple truth!

http://soundofheart.org/galacticfreepress/content/god-gets-his-hands-bible

I'd use God willing? if your mother had a sense of humor. It seems that if uttered out loud that it would come with a very comical overemphasized ironic shrug.

Use the quotation marks ... only if you think it looks better than without. It's a piece of wall art. There isn't a strict rule depicting the grammatical necessity of the quote marks.

I'd use God willing....
(there are supposed to be 4 periods in an ellipsis to a phrase that stops dead and no longer continues)

If you're going to use the quote marks, you probably should add a period before the closing quote mark.
”God willing.”

I bet your mom would tell you that, since it's a phrase leave out any punctuation.