1) The Lakers were shit awful last year on defense (28th in the league). How much of that was not knowing what they were putting out the effort for? I don't know and can't say.
2) KCP may have potential, but is he going to put in all that work on a one year contract with a bad team? I think he's going to want to average 20 pts/game and get paid. But I don't know him, and obviously I don't know what the Lakers plans really are.
Basically, a guy who needs help to be very good is going to a team that hasn't shown much interest in defense (or anything) the last couple of years.
To play devil's advocate, KCP is going to be playing for another contract. I don't think he thinks of himself as a 20 pts/per game scorer; I think he understands his place in the league, and wants to turn himself into a 3&D guy. That's the role he willingly took on in Detroit, and it's unclear to me that he has designs on being anything more than that.
I suspect the biggest cause of the Laker's defensive issues was that they had several bad defenders and combined those guys with several young players who have no idea what they're doing. There are a couple of positions -- namely the 2, 3, and 5 -- where they're likely to see improvement. KCP will be a big upgrade defensively over the minutes they got there from Russell and Clarkson, and at the 3 Ingram's likely to improve just by dint of being a year older, stronger and understanding the system better. I don't expect him to be suddenly elite or anything, but he should prove to be an upgrade over what he provided last season.
At the 5, Lopez isn't some elite rim protector or anything, but he's a slightly better than league average and will be taking minutes from two of last season's worst defensive 5s by RPM in Mozgov and Zubac.
Toss in minutes from Brewer, Nance, and Deng, assume a bit of progress from Randle and I wouldn't be surprised at all to see improvement on that end. Bearing in mind, of course, that improvement can still leave them in the bottom third of the league.