Jargon as your SLP would call it or “silly talk” isn’t uncommon with kids with expressive/receptive speech delays. She may not be trying to be silly; the giggling could be trying to find a way to extend her processing time so she can find the right words. (Some kids will say “um” a lot to do this.)
Try not to let it irritate you. There might be going on more in her head then she can express verbally. The next time she says something seemingly unconnected, you can always respond, “Oh?” Give her time to elaborate or just be silly—not the worse thing in the world.