Is picnic a pronoun

No, the Word 'picnic' is a noun and a verb.

The noun 'picnic' is a Word for an outing or occasion that involves taking a packed meal to be eaten outdoors; for example:

We had a picnic today.

The verb 'picnic' is to have or take part in a picnic; for example:

We picnic at the park south of town.

A pronoun is a Word that takes the place of a noun in a sentence; for example:

We had a picnic today. It was a lot of fun.