I'm pretty sure it looks like that because there is a good possibility it is traced. They had another Rikka animation with doggystyle that IIRC said it was tagged with "traced/tracing" on pixiv, but it got removed, you can still see it on here though if you open their tag.
Honestly I don't really mind it if they admit they do it, and don't try and sell it, unlike certain "artists" on crowdfunding sites like patreon.
