Mr Unapologetic has a good explanation what is wrong with the celebration of -day though he overstates his case.
It would certainly be nice if something more meaningful was celebrated but perhaps we should take every chance we get to popularize mathematics. I am not sure.
It reminds me of the discussion about prizes in mathematics. Are they good because they give mathematics publicity or bood because they give a false impression of how mathematics is practiced? I don’t think anyone knows the answer. Personally I think they fail competely to give any idea of how beautiful and important math is but at least people get reminded that it exists which is nice.