There is one professor whom I wanted to ask for a letter of recommendation for grad school, but I just learned that I got an 80% on the final exam. This is a first-year graduate course, and the exam had simple questions (mostly computational), yet I still managed to mess it up. Very dumb mistakes. Now, it appears that I will still get an A+ in the class due to other things (HW, midterm, etc), however, I do not feel comfortable asking for a rec letter after messing up the final like that. I had this professor for other graduate classes where I got an A+, although I also did not do very well on the exams (about 82-83% on average, yet better than the average student in the class). The professor knows me well since I always asked questions and went to office hours, but because I didn't do well on the exams, I feel like they might decline to write a letter.
Is it still okay to ask for a rec letter after a poor performance on the final?