[Everything about being told "I can feel your heartbeat" like that is exciting to Jiro.
There is literally nothing about that sentiment that she doesn't like.
For obvious reasons, surely - she ascribes a lot of importance to heartbeats for very, very obvious reasons, even though she'd never be caught dead admitting as much - but still.
If her heartbeat was noticeable before...

Her smile starts to waver at the edges as it does when she gets flustered, though, and she leans back with it to match - both arms coming off of her to prop herself up with. Intending to let up on her hold so Momo can do as she needs.]

I haven't noticed any fumbling. Actually, I thought you were acting pretty cool...

[She's been so calm even when they were dumping all their feelings out on the table like that... what about that isn't cool?
(Jiro's also very easily swayed by Momo's demeanor in general, though.)]

But... you're right, too. I can't help but be kind of excited, you know?
I mean, it's you. Even if you were fumbling, I'd be pretty excited.