Is it okay to critique my already published paper? After publishing a paper even with multiple proofreading and rerun the experiment, there is a feeling of "I should improve this" or "this section is so vague, I should explain more" or maybe the worst is when spotting an unintentional error in a graph or data.

Is it ethical If I make some kind of blog or extension about the paper that critique or extend my own work?
I don't want to write the extension in journal though because there is still a lot of work to be done for the next paper.

My fear is it can backfire me instead, if I show what I thought a minor error then the editor or someone read that and try to retract my paper. Or it will become a judgement of my poor work.
In short: no, there's no need for that.

Minor errors, like typos, erroneous labeling, coloring and such, sometimes happen and are not a big problem; think of them as printer's devils. If you think that some parts of the paper could be written better (everything always can be done better!), but the paper got accepted, than don't strain yourself on it - it was peer reviewed after all.

If you found something like wrong numbers put in a table, or a formula with an error (but used the correct ones in the research, so the mistake is only in exposition), then you can write an erratum and point out that


  Fortunately, this was only a typing error, and as such it does not affect any results published in XXX.


Examples of minor corrections:


example 1
example 2


Examples of major, yet not crucial corrections:


example 3
example 4




In a more serious case:

If you found that your analysis is flawed, e.g. claimed that X can be used to predict Y, but eventually learnt that it is impossible and the conclusions of the work are wrong, then you should contact the editor, explain in detail what happened and ask what's the preferred course of action. Maybe you'll be advised to write a comment to be published in the same journal, or indeed the editor might decide to retract the work. But this is then the editor's work to decide what's best.

Example:


example 5




Additional reading:


Elsevier retracting 26 papers accepted because of fake reviews
How to avoid the stigma of a retracted paper? Don't call it a retraction
The famous Lancet retracted article linking autism to MMR vaccines