Yes, currently Manchester City's performance is not as good as the previous season. I don't know what influenced the performance of Guardiola's team. In fact, if you look at the squad, Manchester City can be said to be in good condition at the moment, because the majority of the main players are fit, perhaps only Rodri is injured.
Three biggest reasons: injuries, ageing and less ambition in their players. Injuries are easily to blame as a main reason for their bad performances, it's only one of reasons. Another reason is ageing effect on their key players like De Bruyne, Walker and other players too. A last big reason is lack of ambition and motivation to fight for more titles, after they already lifted many trophies so far. An example is Grealish, after winning 3 titles in one season especially Champions League, he fell to parties and got many injuries, had bad performances for his club. Other players have this issue too, just more or less than Grealish.
Talking about the strategy that Guardiola often applies this season, I think it is the same as in previous seasons. So for some reason Manchester City's performance this season has been a little bad. Because in last night's match, Manchester City couldn't win the match against a team like Feyenord. Even though the match was played at Manchester City's home ground, it was strange that Guardiola's men were completely overwhelmed in this match.
If your team could score 3 goals but let the opponent to equalize within only last 15 minutes of the match, it's terrible defensive performance. Manchester City actually could destroy Feyenoord but they did not do it, and in last minutes, lack of concentration caused this nightmare for them. They are self-making more difficulties for their journey in Champions League this season by this draw.