That depends on what you mean by "long clever words".
I have a book called "Utter Drivel", compiled by the Plain English Campaign, which shows some of the worst examples of bureaucratic English. At the front is a letter the organisation received from Prince Charles. It begins:
"Due to a frequent regrettable inability to prevent my presence in other locations, I find that I must convey to you my goodwill in a correspondence format."
Why say it like that? (Well, in this case, Prince Charles was, of course, joking; but some people really do write like this.) Use a more simple style, and your message becomes much clearer:
"I'm writing this letter because, sadly, I can't visit you in person."
Another danger with using long clever words is that if you don't know exactly what they mean, and you use them wrongly, they become long stupid words. For example, if you describe a person as "perspicuous", that's wrong: it should be "perspicacious". But it would be so much better to just say "shrewd", which isn't so easily confused and which is understood by more people.
But if you go to the other extreme and use only very short, simple words, your writing can end up very dull, like a book for teaching children to read. Simpler words are usually the best, but you do need to make sure your writing is interesting.
In many cases, longer words have very precise meanings when you are talking about a certain subject. For example, if I said "unvoiced labio-dental fricative", a linguist would know exactly what I meant. In day-to-day conversation, I would just say "the letter F". Why use the longer term? Because it describes exactly how the sound is made, even to somebody who speaks a language that doesn't have that sound (Japanese, for example), and also makes it clear that I'm talking about the sound, not the written letter. The long, clever, complicated version is used within its subject, by experts talking to other experts, and it should not be used outside of that context. This is called "jargon".