In the context of being a moderator for a panel of people who have gathered specifically to talk about their things, yes, it's a bad thing to geek out and ramble over someone. The audience haven't gathered to listen to the moderator crap on; they're there for the panelists.

Chairing a panel of professionals isn't about you getting to talk to them, it's about showcasing their positions for the audience. "Aw, he just got excited" is not an excuse in this context.