Is it okay to publish your entire work of PhD in low tier? I am a PhD student works in theoretical computer science(Algebra). I have been working on my PhD project from last five years. I am very much interested in  my work. During my Ph.D there were not collaborator were there other than my research supervisor. I struggle a lot during my Ph.D as problem finding was very tough, solving also and then writing. I even did not find the course work which I did any helpful. I was also not allowed to work outside my university I don't know why. Now at the end of my Ph.D I come to know the value of collaboration.

I have studied most of things in Ph.D. at my own. I come up with results and written those with the help of research supervisor. I have worked very hard in my Ph.D. despite of that I am not publish a single paper in top tier. All of my research paper are in low tier. I will be graduating soon.

According to my research supervisor, PhD is to train a student not to publish something in top tier. This is true for all of his Ph.D students that they publish in low tier.

I am worried about post-doc and Job after my Ph.D as many of the students in my field are publishing at top tiers. I can publish research paper at my own but not in top tier.

Question : Is it okay to publish your entire work of PhD in low tier?
Back in the old days, it was quite common for mathematics and theoretical computer science PhD students to graduate with zero publications.  Their research contributions, while valid, were not significant enough to be published.  This isn't so true now, principally because there are many more low-tier conferences and journals to publish low-significance results, so the bar of significance to be published has gone down.

As far as the goals of a PhD are concerned, as long as you have done enough research, it's a valid PhD.

However, the job market is not so friendly.  It's useful here to bring out the sports analogy.  Getting a permanent academic job is like getting to play on a major league baseball team (or a first division football team).  Getting a PhD is like becoming competent in a minor league (or lower division) team.  Not everyone who starts out playing professional baseball (or football) makes it to the top, and you need talent and luck as well as hard work.

Most people who finish a PhD don't have the right combination of talent and luck to land a permanent academic job, just because there are fewer jobs than people with PhDs.

In the current job market, in order to land a permanent academic job where research is a significant portion of your duties, you need research results that can be published in a higher-tier venue.  If you don't get them as a grad student, you might get them as a postdoc, and it even happens that people get them doing research nights and weekends while working at a non-academic job, or a teaching-only academic job.

But, just as the sports analogy suggests, while there are things you can do that will help your chances of making it to the major leagues, there is nothing you can do that will guarantee (or even make it highly probable) you make it to the majors.