Actually I unironically consider Arnold a good actor (In a very specific way).

If a measure of good is being able to take ownership of a role to the point where the audience can't imagine anyone else playing it...Arnold has the presence and the charisma to do that no matter what role he plays. Sure, his english may be stilted, and he's not going to be winning an oscar or anything.

But acting includes physicallity (IMO) and I can't picture anyone else in a huey puffing a cigar like a badass and then accusing his buddy of pushing too many pencils in the manliest hand shake scene ever in Predator. And then telling a bunch of kindergarteners that It's not a tumor...

Only Arnold can pull that off. He's iconic. And THAT takes at least some ability.