Does crushing a zyrtec pill make it less effective?

No: It is inherently a long-acting drug and thus crushing kicking, dissolving, chewing etc would not affect its effectiveness or duration of action.

No, it's bitter: When you chew or crush the tablet of cetirizine (Zyrtec) it does get into your system a little bit quicker but it doesn't reduce the effective time. It should still give you antihistamine effect for 24 hours.