He is meant to portray him as a detective who is not that good with pretending to be someone else would.
Harrison might or might not be a good actor, but Deckard certainly isn't supposed to be a good one.
Why not? He's a seasoned detective with a track record, why's it so hard to believe he picked up some tricks?
J.J. Gittes is also a seasoned detective, doesn't mean he would be great acting.
Actually, if he's a replicant made to believe he's human, he might even believe he was doing a good job, because he'd be clueless about what good acting is supposed to look like.
Or, if you believe he's human, his the typical hard-boiled detective, he did behave in the way Ford portrays in the movie. Not subtle, not particularly clever.