Go with the professional opinions!

Do you think your dad's comment has to do with you struggling to start writing or completing assignments, etc.? Writers, or artists, sculptors, etc., tend to love what they do and do it whether they're told to or being paid or not. Perhaps that's what he's looking at?
Then again, I've learned that "writing" means different things to different people. To most, it means grammar. It took me a long time to figure out that in the professional world, being a good writer (meaning a paid one) was about the transfer of ideas/concepts in an accurate and/or entertaining way (if you can do both, you can get paid more as opposed to being good at one and terrible at the other). Grammar and punctuation skills are not what you're paid for at the higher levels as a writer or author.
I used to sweat every word, every comma, etc., before submitting my work, but then I realize that I was putting editors out of work. They get paid to do that!
However, I will say that spelling, grammar and punctuation counts in the beginning. Your work will end up in the dreck bin if your work looks like it hasn't been carefully edited and well written.