How to judge the quality of academic paper (students pov)?
First, I think that a paper is a "good" paper if you can learn something from it. It doesn't matter what other, more experienced, people have to say. The reason for that is ....

Most serious researchers in various fields, math in particular, write for a certain audience. That audience doesn't include novices. Papers are written for other people at a similar level of sophistication as the writer and who are able to fill in details of argument without it being explicitly provided in all cases. Occasionally this leads to errors as you might guess, but it is the way of it. 

But, on the other hand, most researchers also include a fairly complete list of references and perhaps an even richer bibliography. This is where the novice, even a beginning doctoral student, has to start. Read a paper that seems interesting or was given to you by an advisor. When (not if, when) you get stuck, go to the references for older papers that contribute to the ideas of the current paper. Start to read and understand them until you get stuck. Then, ... Do this recursively until you reach Euclid if necessary, or until you understand the current paper you are reading and then work forward through the stack (or the tree, actually) until you understand the paper of interest that started you on the quest. 

Take a lot of notes as you do this. Don't depend on memory, especially short term memory to keep you on track. 

You may need to do this process a few times until you become more accustomed to the ideas and gain insight into what is going on in that small part of math. The papers contain a lot of detail, but it is the insight, not the detail, per se, that gets you to true understanding. 

But the notes you take will help guarantee that the whole (painful) process actually leaves you better off at the end. If you actually learn something, you have made a personal advancement. 

I'll note for completeness that some people in science and math do write for a broader audience. Carl Sagan was noted for this, of course, as is Neil deGrasse Tyson. You can learn quite a lot from such authors, but not to the same depth you get from specialists writing for other specialists.