I don't think either of them have looked lost for the majority of the season. Cunha looked fine to begin with, but he's been a walking disaster in the last period. Mbeumo has generally been fine.
In regards to Mainoo, if you disregard his approach to being told to improve, hashtag free Kobbie, i still think there's a huge disconnect between what he's actually doing on the pitch and what the perception seems to be by a lot of fans. How often do you see Kobbie play a pass and move into a position where he can actually receive the ball again? From my perspective, most often he'll pass the ball and jog into a position where he's covered by the opposition. Which is why i don't really understand the notion that he's struggling because it's difficult to play one-twos with Ugarte. Sure, i can imagine more suitable partnership, but Mainoos strenghts aren't exactly built on passing the ball to teammates and instantly moving into a position to receive it again.