Ok to delete these?

You originally found actual core dumps (e.g., core.871) which are indeed memory snapshots of crashed programs but you somehow got lost, ended up in /tmp, saw the "systemd" on some folders you couldn't delete and somehow concluded that those are systemd core dump files. But they're not.

Systemd has a security feature called PrivateTmp. For services that have this enabled (like the bluetooth, colord, and resolved services shown in the screenshot), systemd mounts a dedicated, isolated temporary space just for that service. This prevents services from snooping on or interfering with each other's temporary files.

They have "?" sizes and won't delete because they're are owned by root to enforce that security isolation. Because you're browsing with a file manager, probably running as a standard user, you're being denied access. The file manager can see the folders exist but it doesn't have the permissions to see the items inside (hence the "? items") or the permission to delete them.

Temporary files and these private directories are cleared and recreated fresh every time the system boots or the service restarts so the date says today because that's when the system was last booted or those services were last (re-)started.

You should go back to working on the core dumps. Or just delete the core dumps. But please leave those systemd folders alone. They're not core dumps but a normal functioning security feature, and the OS is protecting them from deletion.