my second reply to you in one day!
I think your sentence is probably OK grammar wise, and sounds fine on second reading. But on first reading, it's a bit difficult because it's a bit long, and the "strolling by" sounds like it might have been done by her instead of you. How about
:
"When I strolled by my new hire's desk on her first day, I was utterly dumbfounded to find her struggling with her calculator as if she had never seen one before. My very first hiring mistake!"
I know that technically my second sentence isn't a sentence, but I think that's OK in creative writing. I find confusing sentences almost always benefit from breaking them into two.
Just my 2 cents.