Until I watched Mammals and read the references to James Corden, I'd never heard of him. I still don't know much so this review is based only on the show, nothing else.
The story is nothing special, boy-meets-girl, fall in love and then out of love (will they get back together?), but the low-key presentation and somewhat laconic humour make it mildly amusing. Corden's strength in Mammals is not humour though, but a searing, almost child-like, emotional honesty. This is more interesting than the somewhat 'modern' reliance on bathos (think The Office, or Fleabag). A lot of what passes for humour here is bathetic, but if comedy is the art of saying something serious in an amusing way, then there is enough here to entertain.
Where it falls down, and quite hard, is the meandering fantasy of one of the characters that doesn't seem to serve any purpose in the story arc.
No one comes out of this particularly well, and I was left a bit deflated and wondering why Mammals had been presented this way (the advertising photo is utterly meaningless). It's okay, but there is much better out there.