I want to be a graphic designer but I’m awful at writing copy, editing, coming up with slogans and catch phrases.
Do I really need to learn Writing, Spelling and Grammar and such to be a successful Graphic Designer?
Required? No. The larger the company, the more likely it is that they’ll have specialized departments for certain jobs. Marketing is a skill, writing is a different skill, graphic design is another skill, web design is a specific subset of that skill, programming is an entirely separate skill.
If you can write, great. In a small shop, that will help you get hired. If you can’t — and by that I mean “write creative business copy,” not “basic spelling and grammar which showed you graduated high school” — it shouldn’t hinder your general employment chances.