We have seen several gamblers try to make a strategy to outsmart this "brainless" game. From that the answer can be seen. The human mind keeps on working even if one doesn't see it. A working mind keeps developing, no matter how little it works, so to answer your question whether playing non-skilled games makes us dull is no.
Requiring calculation or over thinking is the only thing op sees as making use of the Brian in gambling whereas biologically human brains are always in action even sometimes when at sleep, perhaps the games in casino may require little or no effort in playing them yet still retain some level of thought before one goes ahead to make a choice hence there is no moment without the use of brain and not stressing the brain biologically makes it contract and expand yet the use is in place unconsciously