Hes still an elite poacher. Man City need an elite poacher. On the pitch, he literally ticks the box of what they need in attack. Someone to be a threat off the ball, make the runs in behind and find open space. Not to mention his big game mentality and record, and CL pedigree. So on the pitch, yes.
Off the pitch it's a nightmare. This gets them fame in a way no other signing could. It gets them one over United. It pushes United down and cements them at the top in terms of current reputability. It's a slow transition turning into a big club, you always need a lot of convincing to get fans and all that, but a signing like Ronaldo is a big step that would otherwise be impossible.