Yes no doubt effective and qualitative writing demands a mastery over English in all facets, your skill set needs to be upto the mark so that you can effectively express your thoughts onto the paper.

Having said that, it isn't mandatory for a CSS aspirant to obsess with learning English at the highest level thereby indulging in cramming every word there is to know because at the end its not enough, which will only lead to disappointment and further denting your motivation.

What's more of pivotal important is to write grammatically correct statements because they're equally effective. Its not obligatory to write complex statements in order to express your opinions, sometimes simple to the point correct sentences are enough to reflect your thought processes and convince the examiner because in the end that's what counts the most in Central Exam.

Just remember 50 odd grammatical mistakes can cost you essay paper irrespective of the content you worked so hard to write. Therefore it is necessary to focus on your grammar and precise to make your expression simple yet effective and don't worry about the rest.