I think what comes across as bad delivery is intentional uncaring jadedness from Simmons part.

I felt the same way, possibly having been over-hyped by the prospect of hearing a villain voiced by the man who gave us Cave Johnson, and then... meh?

Seeing the behind the scenes footage of all the voice actors in the booth, it definitely feels like he's trying, so maybe the lines are weird or it's just hard to play a guy who is both figuratively and literally dead inside?

I definitely agree, Ketheric struggles a lot with delivery. I think, at the very least, that I expected him to "open the trottle" at the end and finally let loose, but he still felt restrained.