I dunno if this is true. ENFp's certainly don't seem to be the type who takes life too seriously. I am generally a pretty serious person but I know betas who take things very seriously also. It seems like ESTj's and INFjs are more likely to take life seriously than ENFp's and ISTps, but i know that's probably a stereotype. I guess it all depends on what you mean by "taking life seriously." I think that worrying about being productive/useful members of society probably has more to do with individual characteristics rather than personality type, but that's just my opinion.
~INFj