Pharmacists have to read a lot, and they have to understand complicated and dense material. Good grades in English show that you are a good reader and are intelligent. That would definitely come in handy as a pharmacist. It also is required to graduate.
Becoming a pharmacisty is going to require a college degree. Any degree program you pursue in college will require both reading and writing. In addition, we are judged by others, in part, on the way we talk and the way we right. Good English skills are necessary to send others the messages we want them to receive.
In order to be a Pharmacist you are going to have to attend a lot of college. Some of that is going to involve English classes. If you can not get through those you can not get into a pharmacy school. Also as the other poster stated, even as a pharmacist you are going to have to be able to effectively communicate with people.
There are several things to consider about your question.
First, please understand that getting a good grade does not guarantee that any real learning happened. I'm not implying this is the case for you, but it does happen. I would encourage you to make sure that you're not just memorizing to get a good grade, for example. It's important that you actually learn and apply that learning as you continue through to your university studies.
Second, a good grade in English generally means you have the ability to read, think, and write effectively. Anyone who can do those things well (and I would add speaking to the list) will be able to use those skills in every kind of coursework. That's a huge benefit in college.
Finally, having the ability to read and write effectively will specifically benefit both a pharmacy student and a pharmacist. This particular field is time-intensive, so anything that will save time is helpful. Being able to articulate your thoughts for all written work is a natural benefit for a GPA, as well--which will be a significant factor in your graduate school options. Once you have become a pharmacist, these same skills are helpful. The ability to communicate effectively will help ensure customers feel comfortable with you and confident about you.
Best wishes as you follow your dream!