Just use your fingers or a grinder. I also think you'll squash your bud and spread all the trichromes on the pestle/mortar, which would just be a waste.
If you plan on smoking it, then it's a bad idea. Like the other have said it won't work too well if your bud has moisture in it. You'll only squish it. However, if you are going to be cooking with bud the mortar and pestle can be a handy tool. You will want to make sure your bud is crispy dry. You might even want to grind it first and shake off some kif too. Mortar and pestle is best if you want to make a dry powder. And the only uses I can see for a preparation like this is for cooking and possibly vaporizing.