I wouldn't say the Foo Fighters are any kind of Nirvana, let alone a MOR version. There are a lot of significant differences between the two. Probably the biggest one being that the Foo Fighters have a decent number of songs that are listenable from beginning to end, whereas Nirvana only ever came close once with Lithium, but still fucked it up by making the bridge vacant gaze, drooling mouth agape level stupid.