Should I upload Marpipe into a new catalog?

No need to create a new catalog solely for Marpipe. Creating new catalogs just for testing isn't recommended by either Meta or Marpipe. New catalogs tend to have a ramp up/learning period of about 30-45 days. This is because catalogs optimize over time as product level events occur through the connected Pixel. Our goal is to provide creative treatments to these products without losing out on the catalog performance optimization that's already occurred.