Having a great vocabulary is actually great. But if you are going to use it primarily to impress, and not to allow you to express your ideas with better clarity, you got the idea of vocabulary wrong. A sizable population who are applying to graduate programs believe that standard of the SOP is reflected by the standard of the words used, and some resort to the extreme method of removing simple words, and substituting a bigger and more sophisticated-sounding words. If you do this you are not making an impression of maturity or sophistication, as you believe.
Instead you come across as phony, your ideas warped, and your intention is lost in the ocean of complication that you have introduced. One good strategy you can try is to make your SOP conversational. If it appears different from the way you talk with decency and little formality, you have made it awkward and pretentious, at least it would show as so. This does not mean we say you got to forget grammar and type it the way you do with your girl friend on a romantic night. We just say keep it simple, keep it conversational, don’t use a big unknown word when an equivalent small word can be used. Languages become powerful not for the words sprinkled around, but when used rightly and understandably, when words are woven into your ideas with harmony. Even if your language is not great, don’t make it worse through prettifying. Simple words and sublime ideas, grounded on a reality that we can understand, if this is how you represent, your words are too powerful to be ignored.