Don't know about the face she made, but the "not" in the end is the joke.
"Is it true?!"
"Of course.....not".
That means, Manana do think nobody would fall in love with her.
Anyway, there's no yuri, and if there ever is, it would be for comedy sake. As always with nonsense gag manga.
Not having read the source material, I can't tell for sure, but seems to me like it's:
"Nobody will ever love me."
"That's not true."
"Really?"
"Of course not."
The 'not' isn't in response to the 'really,' but to the 'nobody will...'