Are body rows a good exercise if you can't do many pull-ups?
They work a different area of the back. Whenever I train back I do exercises for back width and back thickness. Pull-ups are a back width exercise. Rows are a back thickness exercise.
Back when I was around 20–21 I couldn't do many pull-ups either. I could maybe do 4–5. I just used the cable pull-down machine, but it wasn't giving me the big V shaped back that I desired. Then I got a new coworker who had a great V shaped back. Even relaxed it was a total V. When he spread his lats he looked like a cobra. That's what I wanted! So I asked him how he got such great back muscles development and what I should do.
He said simply, "do 50 pull-ups every time you do your back workout."
I replied, "I can't do 50 pull-ups. I'm lucky if I can do five."
He says, "You can do fifty. Do your 5. Rest. Do 5 more. Rest. Do 5 more. Don't stop until you get 50."
Then I said, "it would be more like 5, rest, 4, rest, 2, rest, 1, 1, 1, 1, 1, for an hour!"
His final response was what convinced me. "You asked me what to do. That's what I do. I can do my 50 all at once now, it's my warm-up. But in the beginning I was where you are. If you want it as bad as I did, that's my answer. Do it or don't, I don't care. I already have my back."
He was right. And I took his advice. It took my doing those 50 pull-ups 2X per week for three years to get so that I could do all 50 at once. I continued making the 50 pull-ups my warm-up on back day for the next 20 years.
Here's a picture of my back when I was in my early 20's (left) and again over 20 years later (right).