Gertrude has pointed out to Hamlet that death is a natural part of life, and Hamlet agrees. She asks, and I paraphrase: So, if you know death is common, why does it seem to be affecting you so particularly? He is irritated, firstly, that she says "seems" because there can be a big difference between what something looks like and what it is. He is genuinely this upset about his father's death. He is particularly affected; he doesn't seem to be.
Then, he goes on to list the trappings of mourning -- dark clothing, tears, sighs -- and he says that a person could fake grief by using these things if he wanted to. We know that Hamlet is angry at his mother and stepfather/uncle for marrying and especially for marrying so soon after his father's death. In saying this, he is implying that her grief really was not sincere; she may have worn black and cried a lot, but the haste with which she remarried proves that she didn't truly grieve Hamlet's father that sincerely. With her, it "seemed" as though she grieved heartily for a while; with Hamlet, he truly does grieve still.