That's not a very good idea. Most people are terrified of glass floors in other scenarios, like balconies for example, especially combined with some altitude. It may be just as safe as "normal" windows but I doubt most people would be comfortable with it. I'm not even sure I would be and I have no more fear of flying than most people. I'm not sure it's suitable for other reasons as well, such as hull integrity (but I'm not an engineer), or actually getting sunlight into the cabin. I'm fairly sure most people want to watch sunsets and sunrises, not clouds or how insanely high up you're sitting.