hi,
robovm compiles java bytecode through LLVM into native code.
the question of decompilation is similar as decompilation of native code.
in general its question of reading of decompiled assembler code and converting it back into some readable form.
(e.g. in general not easy and not one click operation).

as a note RoboVM binary contains information such as method names and line number tables that is used to produce exception stacktraces. this information would make decompilation process a bit easy.

in general -- any program is decompilable its only matter of time, experience and money to be invested into :)