Contractions ("can't", "don't") in a mathematical paper
My experience has typically been that the more formal mode, with no contractions, is preferred for most scientific publications.  It is not a terribly strong or important custom, however, and in one of my more high-profile multi-disciplinary publications, I actually found the copyeditor introducing contractions into my writing!

In short: worth mentioning, but not a big deal.  As a reviewer, you might say something like: "I found the contractions distracting, and would advise removing them" but I wouldn't recommend being much stronger than that in your statements.