Okay, so the system is reporting the entire system's load inside the container - there is no way the container itself can find out its own CPU load. Likewise, your htop screenshots show what the container sees two cores are around 25% each while the only process using noteworthy CPU% is perl, pihole-FTL and htop merely contributing to like 4% (instead of 26.4 + 28.7 = 55.1%).
What is missing here is a screenshot of the web interface but I guess it will have shown something between 1.06/2 = 53% and 1.51/2 = 75% at that time. We could indeed change this to use the number of available (nproc --all = 4) instead of configured (nproc = 2) so the shown percentages would have been between 26% and 32% matching what we see in your htop (27.5%) and top (17.8 + 10.0 = 28.8%) screenshots.
FWIW, that the container does not know its "own" load but also sees the entire host's load seems to be an LXC bug and one that will not go away soon, too. It is the same for docker which does make sense.
Please run
pihole checkout ftl tweak/nprocs_conf
and check if the CPU percentage is now what you'd expect. But please keep in mind that the CPU utilization inside LXC containers will be the same as on the host system due to the aforementioned LXC bug.