They're kind of obscure. How many non-comic readers would recognize Howard or Dazzler? On the other hand, even most people who have never read a comic book before will easily recognize someone like, say, Batman or Spider-Man. Even some comic readers might not be familiar with Howard or Dazzler. Granted, they're not as obscure as someone like Looter/Meteor Man, Man Mountain Marko, or Plantman, but they're not exactly household names, either.
As for Howard's movie appearances...yes, Guardians of the Galaxy was deservedly extremely popular and no doubt gave him a lot of recognition, but I don't think that many people remember (or want to remember) his feature film.
But then again, how many non-comic readers do you think saw GotG and recognized Howard? Not very many.