Is it okay to add your own properties into the 'SC' object that is globally accessible? (I'm developing extensions for SC 2019.2)

It's not great

I want to add some common properties that need to be consumed by Views from multiple extensions. I cannot fire queries to the back-end to refetch that info for every extension. What will be an ideal way to share that data? (Something like the SC global object.)

Depends on your data. We're pretty keen on using cached models and collections and then returning them as singletons

Yes, we're using the Configuration record for storing static values/configuration. But the info I'm trying to make global is dynamic (it should be fetched once every time the web store loads - a particular URL) and then made available for multiple Views.