Is it bad etiquette to decline a draw offer when participating in a simul? On LiChess, FM knightprince gave a simul where I participated (our game).  I blundered a pawn played a pawn gambit, but managed to complicate things and won it back.  Afterwards, the game would have led to a drawn endgame.

The FM offered a draw; I wanted to refuse, but thought that would be perceived as ungrateful, so I accepted.  I wasn't expecting to win; I just wanted to play out the endgame for the experience.

Question: Is it bad etiquette to decline a draw offer when participating in a simul?
No it's not bad etiquette at all. A player being higher rated does not entitle them to automatically get the result they want, even if the position seems to indicate such a result.

And in the rare case your opponent gets offended by this, congratulations. You've just gained a psychological edge.