That is so not true. What is easy is to use the basic theme to build out a prototype of your app. Customizing will always be harder, but then again, it's been hard to build out a well-designed site since the very beginning.
To say that it's hard because it comes with a default theme is to not understand.
If it makes it easier for people to communicate faster, than I'm all for it.
I am sorry, I am not a designer and not a native English speaker, this makes it a bit harder to formulate this correctly.
Then why would you even be talking about Bootstrap, let alone criticizing it from a design point of view?
You can take any past, present or future "original" concept and build it with Bootstrap. You do understand that, right? You are not limited to the default theme. If you're lazy, lack talent or interest then yes, your website will look pretty much the same as those of your peers, but then you can only blame yourself, not a CSS framework.