If you delete all your posts, yes your activity will go back to zero.

Activity works by taking the number of posts you have made within a 2-week period (capping at 14) and adding it to your current activity, which starts at 0. These values are stored in a database and updated with a script that runs periodically every other Wednesday.

So mass-deleting tons of your posts will lower your activity and consequentially your rank on the next Activity update cycle.