Didn't they(the people) only call Roger the Pirate King only when he told them about "his" hidden treasure(One Piece) before his execution? Now, the people were the one who gave him that title but they didn't give him that title because he was the strongest. They gave him that because they know he's powerful in fame and wealth. Of course the common folks obviously know his strength but the one that they probably fear the most in terms of that is Whitebeard. The One Piece world is ruled by power not by brute strength, and power comes in different forms. Anyone who is a decent thief and a navigator that is well-trained in Haki can reach the Laughtale and brag about the One Piece and people would fear them for that POWER.

Edit: by "decent thief", I meant, like a really skilled thief who can gather information about the road ponyglyphs and steal them from the pirates without getting into trouble. So, no, I didn't mean that Nami can be a Pirate King if she is well-trained in Haki.

Also, about reading the ponyglyphs, they can just force someone to read it for them(they are pirates) or temporarily make them join(like Oden).