Marigold looks absolutely adorable in this strip. I'm not quite sure about the bikini though, given Marigold's body type I would think she would be bit more plump (though not obese) whereas today's strip makes her look curvy with little extra fat, at least to me.
This strip definately marks the possibility of Marten/Marigold happening. We did speculate that Marten, as pseudo-main character might end up with someone seeing as this is major arc, and Marigold is available. Also, Marten wouldn't be likely to hurt her in a relationship. It all depends on how he and Marigold feel about it. In particular since Marigold can be/is very clueless about relationships.
And, finally, I don't think she realises this, but, Marigold made a guy flustered based on her looks, which, knowing her and her history, I can't imagine has happened before. It might be considered a milestone, though, just the fact that she walked out in that bikini with her self-confidence problems is a pretty big thing. If she actually goes out in it to the lake it will be a definite step forward in her character development. Because she isn't as strong as Hanners (personality-wise) she hasn't progressed as much as Hanners has, so it's always nice to see her take steps forward. Also, she looks really cute in the last couple of panels as her blush darkens while Marten is getting more and more flustered.