Here "generation" refers to the people in it. AT: "The people living at this time are evil people. They seek ... to them" or "You people living at this time are evil people. You seek ... to you"
The information about what kind of sign it seeks can be made explicit. AT: "It wants me to perform a miracle as proof that I have come from God" (See: [[rc://en/ta/man/translate/figs-explicit]])
"what happened to Jonah" or "the miracle that God did for Jonah"
# For just as Jonah became a sign ... so too ... this generation
This means that Jesus will serve as a sign from God for the Jews of that day in exactly the same way as Jonah served as a sign from God to the people of Nineveh.