Probably. I don't think he has fallen off the cliff. He is usually in the right place and contests shots. However, against bigger guards he is just to small to bother shots. I would say he is average at this point, or even below average. He is less 'switchy' than say Rubio or Hill, so I see how we lost some flexibility there.
I think Zanik/DL made a calculated risk this offeseason: trade defense for playmaking and shooting after what happened with the Rockets. Heck, Toronto just won a title with a tiny backourt of Lowry/Van Vleet. They overcorrected.
The problem? We lack other defenders that can allow us to hide one or even two players on defense (unlike Toronto). We over rely on Rudy. And the rest of the cast has been underwhelming all year. Ingles has lost a step. Bojan and JC are below average. Bradley is slow. Niang, well, is Niang. Royce is the only good defender but even him is getting cooked now. Davis, Green and Mudiay gave us nothing.
We addressed the shooting but lost our defense (and identity) big way. I'm ready for next season and the FO to find ways to address this: one or two capable wing defenders and a back up big. With Bojan, DM, Conley and JC we have enough firepower to go against any team. No defense though.