Yeah, we couldn't figure that one out either. Other than it (Howard) was part of the Collectors "collection".....very odd. The whole theater sat and waited through the credits only to be let down by such a ridiculous scene. On top of that, some girl in the front of the theater yells out "holy $&I! it's Howard the duck it's Howard the duck" ! I don't know if she was trying to be funny or what , but nobody laughed. We all walked out disappointed having waited through the credits for that crap.
Yeah that spoiler was eh. For those asking Howard the duck is part of Marvel's collection and it was meant to be a goofy spoiler. Had this movie been made before we saw marvel kick out spoiler after spoiler that got fans excited for the next movie people would have been ok with it. But because Marvel raised the bar with teaser credits people now go into the movie waiting to see it. Had the movie been not as appealing with the test audiences I think we would have seen more (ie antman, avengers, black panther) but because test screenings were full of positive buzz marvel probably feel like they have built up so much credibility in the bank where they can now have a better control of when to release clips as they do not want to build up hype too soon.
Have no clue, I too am new to the GOTG franchise. I had to wiki some of the things I saw in the movie just to catch up. I feel like Marvel did an excellent job taking an unknown property that no one cared about and building it to a point that people want more.