What do you mean? I'm here.
No, it is not common for the head coach to sign those forms. However, those waiver of liability forms are pretty much worthless, anyway, even if the parent does sign them.
With the roster forms, I suppose if a coach signs for a player or parent, the player can rightfully claim she is not part of that team's roster, or an opponent could claim the player is illegal if they could somehow prove the signature was not valid.