Is there a way to find out what are the most used fields and forms in a Marketo instance?
Depends. What do you mean by "most used" fields? Most changed you can get with an Activity Log extract, looking at Data Value Changed. Most non-empty you could do a massive lead extract, or probably a series of extracts with 100 columns at a time. But most used as far as appearing in smart lists, flows, email content, webhooks... I don't think there's any way to get that, since some of those references aren't even counted anywhere.
Most used forms is straightforward because you can do an ActLog extract and check for Filled Out Form activities.
Under admin > field management you can check for "Field used by" on a field-by-field basis, but again to Sanford Whiteman's point, more clarity is needed with the question.
Retrieving data ...