GiovaneDinamitardo:
I think that Lamù fell in love with Ataru from the first moment he saw, I mean when she entered his house (breaking down the door with a lightning bolt) before the competition began.
The fact that every time he gets up after every disaster and that, at the end (based on a colossal misunderstanding), he asked her to marry should have strengthened her feelings.
UruseiNeo:
At first sight, probably not. Given the kind of character she was at the time, she probably would have either tried to make him her husband then or bribe him with an "easy victory" in the tag game in exchange for being her boyfriend/husband.
There's not really any reason for her to have love at first sight.
I think maybe during the tag game she was having fun with him, she was certainly enjoying herself, so when she thought he asked for her marriage, she wasn't as inclined to deny him. I think she REALLY started to love him when he was rejecting her to live with him (kind of the "wanting what you can't have" kind of thing).
cata:
None.
She began falling for Ataru on episode 10, just like Ataru. In episode 1 she's just one of those immature girls. You can tell that from her facial expressions when the press is interviewing Ataru.