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.

First, I'm not sure where to draw the line between top and low tier journals. You want to publish in an appropriate journal at the higher rather than the lower level.

But it is more important to realize that your career doesn't begin and end with your doctoral research. It is normally only the first of your published works, not the best, and certainly not the totality.

Some people are lucky in their studies, having been given a hard problem and having made significant progress on it. Others have done more modest work. But, there is actually more valid doctoral research than the top tier journals could possibly publish.

So, it isn't something to worry about. But seek out interesting problems and work hard to solve them. Get as much published as you can and in the best journals you can manage. Your career will grow. It doesn't start out fully formed.