My guess is that in moderation reality TV has no effect either way. Like most things. Some of the dumbest and some of the smartest humans I’ve ever met have been fans of reality TV shows of various stripes.

If anything, in small doses they are likely mildly palliative but it’d be hard to measure. And very individual-specific (I find them all annoying and they make me low-level angry).

In large doses, of course, yes reality TV shows make you dumber if for no other reason is that time devoted to one activity cannot be used for anything else.