Franken seemed perfectly lucid to me.
I thought this as I was watching and apparently a number of other people did too. I also have friends who wondered the same thing. For others here, watch the example in the link and tell me if he's lucid. "When you’re a parent, you often have to prepare food for kids. But, if you’re not- if you’re lazy about it, you can put it in the microwave, and the 7-year-old knows how to do that, and I’m saying it’s scientific fact, which it really isn’t, but we all know it is, and then the scientific fact, I went even further with the joke, and you know this, that it’s scientifically proven that a … 7-year-old can teach a 4-year-old."
This is a mild example. If it was supposed to be a joke that he's totally lost any comic ability (though I've never liked him either as a comedian, politician or pundit.) One friend speculated he had dementia, but I attribute it to alcohol because he didn't strike me as high. I almost thought it was possibly a mini-stroke, but we haven't heard anything so probably not.