A mixed reference can mean a few things. I had a career counsellor once tell me that if a reference wasn't completely ecstatic and over-the-moon about you, then it's a bad reference.

So possibly what happened is that your reference said something along the lines of "I worked with anonnystate and he came on time and completed his work". Which sounds good to you, but to a potential employer, this is pretty mediocre and might indicate that the reference is hiding something. If your friend doesn't know how to give a real reference, then this may be the case.