I think she was just happily embarrassed ^^;. She's already giggling before the spoon got anywhere close to her and you can kinda hear Haruna egging them on in the background. Ayumi is just as much as a Sayu fangirl as Haruna or Mizuki or heck, pretty much the rest of the girls but she and Kudou always give some of the best reactions when Sayu teased(flirted with?lol) them
Btw that looks like a normal strawberry ice cream to me.