If you're concerned about something appearing to be too "fiction(al)", then don't go see a movie about a silver-coated alien flying around on a surfboard.
There's nothing childish about Galactus. If people decided to knee-jerk, then that's their fault. We all know who Galactus is.
The problem with these filmmakers is that they care too much about attracting and pleasing the kind of fan who will have forgotten the movie by the following day; good or bad. Fans are the ones who will offer the most stern, continual critique.
-AC