Is it OK for me to write insults about Twilight while reading it for a friend's sake?

Not to her, it would be rude.

But seriously, if you're reading it for the first time, you should put an actual effort into keeping an open mind and done read it with the mindset that you need to find something to insult.

YES!!!!!!!
who does'nt?

that's your choice. It isn't a crime.

If you think they'll find it funny? Go ahead. I had to do the same just to get through the thing without throwing my kindle against the wall in frustration.
If it's to insult them, then no. Leave it out, even if it's just good-natured ribbing; no need to stir trouble for the sake of it.
I don't know you or your friend, so its up to you; just giving my thoughts.

Yes it's your opinion