Generally it is bad practice to plan your strategy around the fact that your opponent is weak.
In multiplayer this isn't strictly true. For example, it makes a big difference in three player games whether two players are buying provinces and one player is buying alt vp, or whether one player is buying provinces and two are buying alt vp. There isn't a single optimum strategy without knowing how your opponents will play. Part of your judgement may need to be "I would do this, but this other player isn't good enough to think of it so will probably do that instead".
I can also remember way back to Theory losing some competition with a 4 player final. There was a kingdom with some fishing villages, island, seas hag, ambassador, and Theory did the sensible thing with ambassadors while the others bought sea hags or islands. The sensible thing lost to the person who bought islands and scored some points before the islands, curses, and fishing villages ran out. In this case Theory needed to plan the strategy around the weak decks the opponents were actually building.