“No One Is Perfect” Is Not An Excuse
Instead, it is a saying that is often misunderstood and harmfully misused.

No one is perfect…
You and I and everyone else are deeply, inevitably flawed.
This is a natural part of being human. It is impossible to live a life without making any mistakes or having any shortcomings: after all, without failure there is no success, and without flaw there is no potential for growth. What this saying (“No one is perfect”) means, therefore, is that it is pointless to chase perfection in performance or appearance, because it is beyond reach.
However, this quote is often severely misconstrued.
The saying is not meant to be used as an excuse for wrongdoing. Despite the actual meaning behind it, “No one is perfect” is often used as a way to evade criticism and accountability, so as to be able to avoid the hard work and humility that comes with growing into a better person. This is ironic, because while the quote is meant to merely explain the presence of flaws that are natural or that we have little/no control over (like shortcomings that are superficial or harmless, or mistakes that we have made in the past), the quote is often used to protect the current self from rightful responsibility and blame.
None of us will ever be perfect, no: but we all have the potential to be good.

…but everyone should strive to be better.
We are all flawed: and that is precisely why we need to change.
Flaws (and by this, I mean actual, harmful flaws, and not superficial ones) are there to be acknowledged and improved upon. Rudeness can make way for kindness, and narrow-mindedness for empathy and understanding. A shortcoming is not merely a sign that you are human: it is a sign that you have areas to work on in order to become a more fulfilled person.
Indeed, being flawed is not a reason to give up and settle for a flawed life.
No, you will never live a perfect life, but you can make the way for a good one (and a much better one at that). Therefore, you cannot excuse your poor behavior by claiming that “no one is perfect”, when you absolutely have the power to become good. If you do, you would be using a saying that only seeks to explain imperfection, to give yourself reason not to grow upon your real faults.
So no, “no one is perfect”, but “everyone should be good”.
And though you will never be perfect, you should always become better.