“You should always improve yourself” does NOT mean “you are not good enough”. Think of Elon Musk, he is successful of course (because he has multiple successful companies), does this mean that a person who is the CEO of only 1 company a failure? Of course not. There are levels to success. In fact, each person has his own definition of success. The point here is that you should always seek improvement whenever possible to reach your goals and your definition of success.
Discussion
"So if you want to be successful, you should always improve yourself", however, does. "You should always improve yourself" is simply telling me what I should do (just implying that I'm not good enough).
Who is to judge what success is? Ideally the person themself, in my opinion.
Meaning every person has a hard line, conscious or not, between success and failure. And of course a continuum on each side of the line. And the line is not necessarily static over time either, but can move in either direction, depending on many factors, both internal and external. Whether the person is good enough to be, or get, above the line or not, is what defines whether improvement is needed or not. It's even (highly) conceivable that in some cases the reverse is true - reaching above the line requires to not waste time improving yourself, time that should have been used to work, or whatever else that the person needs to do to get/stay above the line.