Would it be a good idea to take antidepressants before geting a piercing or a tattoo? Would it help for the pain?

No. An antidepressant will not lessen the pain from a tattoo or piercing. Some people take an non-steroidal anti-inflammatory med before hand. Auricular acupuncture with asp needles can also be used.
No. That is an inappropriate use of an antidepressant.