Why can't I used lined notecards to write thank you notes?

Not note cards as in note cards that are lined with the blue lines and such. Note cards as in cards designed for notes.

I’m writing thank you notes for my graduation presents that I received yesterday. Earlier in the week when I was talking about it with my adviser, we had a disagreement. Here’s how the conversation went.

Me: I’m going to look for lined note-cards to write my thank you notes on. It looks nicer.
Her: You can’t do that, though. You have to use normal cards.
Me: Why? It’ll look stupid when my writing goes diagonally along the page.
Her: You just can’t, it’s not professional.
Me: But its not professional to have diagonal thank you notes either.
Her: It doesn’t matter – you’re not supposed to do it.

Why can’t I? At this point I’m writing the letters on the window with the light shining in, with a lined piece of paper under. It’s really troublesome, if you ask me…

At this point, she won’t be getting a thank you note that’s lined or unlined. :|