It's extremely distasteful and should be shamed.
The reason it’s on GitLab is because it was a free and easy place to put code in the first place.
Mirroring was extremely common when the internet was slower.
There is absolutely nothing unethical here in relationship to the project owners themselves.
This possibly might be damaging to GitHub themselves but even that is a stretch.
I would be extremely surprised if you’ll find project owners that are “offended” by this for lack of a better term and have published their code explicitly on GitHub in order to support GitHub.
It’s one thing to fork, but then you have a different name attached to it. It’s entirely another to clone the open source world en masse with no auditable trails or connection to the original authors.
It could disrupt the community because for issues and pull requests created on GitCode, the original maintainers are likely not going to receive any notifications and they will just be ignored.
GitCode also did not make it clear that those repos are mirrored from GitHub in an obvious way, especially on the organization or user pages, e.g. https://archive.is/su9h5. IANAL, but this looks like impersonation to me.
A sibling comment also mentioned that CSDN is publishing machine-generated blog posts about cloned projects with a link to GitCode. I believe this is even more unethical.