....Goofy??
"Perfectly fine" would probably be a stretch. I've seen lots of people use mobility devices even though they could walk but still suffered from chronic back pain or other various health problems that make standing or walking for long periods of time a real struggle.
But I'd say laughing is okay in this case since it's silly to think a wheelchair could work with a regular escalator.
Yeah, I'm pretty sure every escalator has a sign warning against wheelchairs, strollers, etc. because of morons like her.
Also, you're right on the mobility thing for the most part but I've also just seen people use those scooters as an "I'm too lazy" device. I also sometimes wonder if they do more harm than good in cases of fat people who have trouble walking because they're fat.
But I digress...