Yeah, firstly, you should be betting enough pre-flop to be heads-up at the flop against a weak player. 2 weak players is okay, but any more than 2 and you can't even be certain when you do hit the flop if you have the best hand because the fish could have hit any 2-pair, set etc. far too easily.
Anyway, donk bets generally mean they have hit something here. Even if they have bottom pair, or a low pocket pair, riding 75% bets just to go to showdown with Ace high is still a terrible move. They likely aren't folding to a bluff, or a semi-bluff/feeler either.
The only time I'd consider coming over the top is if I am heads-up and have another draw of some sort and the donk bet is incredibly weak (below half the pot is generally a massive tell they can be pushed off the hand). But you do need a bit more information before you can be certain of these things.