23:1. One is allowed to enter a cemetery while he is wearing tzitzis, so long as he is not crossing over graves [lit. "the tzitzis are not dragged over graves"]. However, if he is crossing over graves, then it is forbidden [for him to tzitzis] because it would be (1) "mocking the poor." All of the above was said during the days when tzitzis were placed upon garments that were worn (2) as regular garments [meaning, not specifically in order to be wearing tzitzis]. In our day, we wear the entire garment in order to fulfill the mitzvah, and therefore it is forbidden to to show the tzitzis even if one is not crossing graves. This is true only when the tzitzis are uncovered [outside of the shirt], but if (3) they are covered (inside of the shirt/pants), then it is permitted to have the tzitzis on.
MB 1. Mocking the poor - For it appears that he is taunting those who cannot fulfill the mitzvah (of tzitzis).
MB 2. As regular garments - For at that time the custom was that their clothing was made with four corners [regardless of the mitzvah].
MB 3. They are covered - Like one who hides the tzitzis of the large tallis under the corners of his garments. Similarly, if one wears the smaller tallis under his shirt, there isn't a mockery here since the tzitzis are covered - unless he does not wear an outer garment [thereby leaving the tzitzis revealed], in which case he cannot wear the tzitzis unless he pushes them up into the corners [of the tzitzis, or his shirt or pants]. (From the Pre Magadim it sounds like one should be stringent with a large tallis, even if covered, since it is reserved for prayers. However the Bais Yosef does not seem to agree, and so too the Derech HaChaim does not say anything about this, and merely rules that if it is covered, it is permissible.)
23:2. There are those who have the custom of tying two tzitzios (plural of tzitzis) of two corners together when they enter a cemetery, but (4) this practice is no help.