is it possible a custom field Comments in a form will go to lead activity log?

Mandy,

Use the method described here to keep track of all of your comments; keep field size limits in mind.

http://community.marketo.com/MarketoArticle?id=kA050000000KzejCAC

Hi Mandy-
In addition to what Jason listed, you could have a comments field that is unique to MKTO (for use in your form submissions), and then utilize a smart campiagn to insert into the Comments field IF blank and then do an interesting moment to the lead/contact record for all so that if sales wants to append the comments section they can.  Additionally, by doing this, you could technically "wipe" the MKTO only field so that if someone navigates back to a form, their previous answer does not show since they are cookied.

Hope this helps.

Thanks Jason and Brigid for your answers. I might give it a  try. The only concern is field size limits. The users could fill out the comments often. That's the reason we would like to make the comments as activity logs. So ,we can view the history of the their comments.