I'm not sure the "dead last" thing is right - he didn't have enough targets to show up in the rate stats (
https://nextgenstats.nfl.com/stats/receiving/2019/all#average-separation) and there are WRs with less separation than his reported 2.2, including Kenny Golladay (1.9), Mike Williams (2.0), Terry McLaurin (2.1), Devante Parker and Preston Williams (both 2.1), plus guys like Darius Slayton, Julio Jones, A.J. Brown, and Allen Robinson also at 2.2.
I don't think Harry is ever going to be great at separating. But his college tape was full of circus catches and we've seen some of his after-the-catch skills and blocking ability. So I think the case for Harry is a guy who can get open enough, outjump and outcatch contesting defenders, make some plays after the catch on some schemed touches, and blocks his butt off. I think the keys for him are improving his press releases, which will let him make more plays downfield, and using his body better to shield defenders.