After some testing, I found out that cffi typenames don't collide with CLOS classes and defstructs. So distinct type name is not necessary.
Regarding "standard for type names" in general, lisp has separate "namespaces" for functions and types. (to be precise, same symbol can be used as function name and as a type name, as far as I can tell).
Conclusion: It looks like distinct naming scheme for cffi types is unnecessary.