Well that would depend.
How fluent are you?
Because the answer to the second question is a pretty good answer to the first.
Obviously, your essay should be well written. It should not sound like your Line messages, or even your AWA essay. An admissions essay you spend 6 months working on should clearly be better than one you wrote under timed test conditions. But it should not sound like the queen of England wrote it, which is what a lot of people try for – consultants included.
There’s a simple reason for that last one:
Editing is harder than writing.
As a result, the people you hire to help you will often write beautiful essays because it is easier than editing your essays. But that’s a bad habit. Not only is it unethical – it’s impractical. The schools have your verbal and TOEFL/IELTS scores. They know where you went to school. They have your transcripts. They know whether you should sound native, and they definitely know if you should sound like a literature major (you should not).
So you should sound like the best version of you. Obviously, you should check your grammar, and you should use the lessons you learned from GMAT Verbal (like the not only/but also rule I ignored in the previous paragraph). You can even use a thesaurus. But you should sound like you.
Here’s the good news:
Schools want you to sound like you.
One, this isn’t a writing contest. If it were, Harvard would be full of novelists, not Investment Bankers. Thai engineers would not be going to Top 10s (which they do). MBBs would be everywhere (they’re only almost everywhere). But there is diversity in these classes, which means there is diversity in language level and style. Again, be the best version of you. Focus on the story, not the literary merit. Don’t let some invisible “editor” hand you Shakespeare. You will not get in. Ad coms are quite smart. Most importantly, they know if you’re willing to let someone else write your essay, you may very well be willing to lie about your achievements. Don’t do that either.
It’s okay to be imperfect, because perfection is implausible. You will never use as many articles as a native speaker, because Thai doesn’t have articles. You won’t say “that” every 10 words, which is actually a good thing. You won’t use idioms constantly, because how could you possibly know that a bird in the hand is worth two in the bush? And is it? I’ve never been sure.
That being said, you should not have comma splices. You should know parallelism. You should probably use adverbs appropriately. You should look like you tried, and like you belong in a great school. There’s a balance. And the best way to achieve that balance is to be the author of your own stories. And only then should you edit until you reach the highest level of reasonable excellence.