People seem to be discussing this in various places -- and in rather heated terms. I'll put my impressions here.
I didn't have the strong reactions many of you appear to have had, negatively or positively. Part of this is my general feeling that Mike Leigh's films (Vera Drake excepted) are too half-hearted to really evoke outsize response. He frequently creates episodic narratives that don't build enough for me to become truly engaged in them. Topsy Turvy would be the epitome of this trait: it had a lot of scenes that were, individually, interesting, but they failed to connect to one another -- stories/relationships were raised, then cast aside. It was as if the script had been written by the Guy Pearce character in Menento. Even as late as the final scene in the film, Gilbert and his wife had a confrontation that came from absolutely nowhere. It was a fascinating set-to, but, that late in the game, what could it mean? There's something to be said for structure, and narrative pay-offs.
I had something of a similar reaction even to the clear best scene in Happy Go Lucky -- Marsan's inappropriate but somehow understandable blow-up near the end. It was powerful, and it did have at least a connection to scenes that had preceded it...but in the end I wasn't sure what I was supposed to take from it. It felt climactic, but not a fitting climax for the full two hours that had preceded it. because, apart from the Marsan thread, the rest of the film was as episodic as Topsy Turvy -- the main character just bounced along, mostly oblivious to others, showing no self-insight. And, though she showed some reaction (at last!) when confronted with Marsan's unseemly rage, a scene later in the rowboat she seemed back in her bubble. I had no idea what Leigh wanted me to feel about all this. What were those two hours about?
Which brings me to the character of Poppy. Everyone here seems to have bought into the "she's just someone who wants to bring a positive attitude into the world". I know that's what the film's publicity promoted (even my Netflix sleeve), and the trailer as well, by highlighting the film's near-last line. But that's not what I felt about her watching the film. Especially based on her first scene with the bookstore clerk, and her initial driving lesson, I thought, This is a person desperately craving attention who's not 1/10 as funny as she thinks she is. Honestly, what came to mind was the Ricky Gervais character on The Office. If I'd been trying to teach her something and she'd pulled on me what she pulled on Marsan, I'd have whammed her one (or prescribed Ritalin). How this translates into wonderful sunny human being is beyond me; it's close to the sentimentalization of mental patients we saw during the flower-child era. A sense underlined by the scene with the homeless guy. The fact that he never seems to frighten her -- not giving a hint of going medieval on her -- makes me feel Leigh wants you to accept that she exists in some state of exalted grace protecting her from harm. This is not to say such a character as Poppy isn't interesting. There'd be plenty to explore in the character I saw. But if that simplified explanation for her is all that the movie's offering, I have to say no sale.
I can't say I'm surprised Hawkins won some critics' awards. It's a dominant role, a relatively original one, and Mike Leigh's actresses have been well-rewarded in the past. But it baffles me that, in a decent year for actresses, all the reputable critics' groups came down on her side. 2008 was a better year than her clean-sweep would suggest.
Marsan was exceptional, and clearly merited a spot in the supporting actor ranks.