First off - DO Not use clay bar, just because. Unless you willing to follow up with a complete machine polish/correction. Although clay bar with subsequent machine polish to prep is great and necessary-read on the third paragraph.
Secondly, since Hatchback style rear end exposes to different turbulence scenario comparing to other design style. Exhaust fumes and particulates as well as other air borne contaminants can deposit onto rear bumper and even vertical surface. It is not unique on your Hatchback! In the Corvette world, C5/C6 in particular, bumper rear end is one vertical surface, which has been a nuisance since the beginning of production.
Aside being unsightly, and to combat possible eventual damage, as well as easy maintenance, it is best to prep surface and apply protection. Long term protection can be using expensive film or cheaper ceramic coating.
On a personal note, my C6, I first ceramic coated it on 2009 and the second application was 2018 and my wife's 2018 Hatchback Sport was coated same time. Maintenance consist of wash, follow with no touch blow dry, using MasterBlaster and after, simply walk away.
Sorry for the long wind, but the moral of the story is not to simply using clay bar w/lube!