In OTB, there is a rule that you can claim draw if your opponent is not trying to win "on board", regardless if the position is equal (theoretical/tablebase draw) or lost (and the opponent has no counterplay).
Obviously, this is hard to always consider exactly, but this rule can apply in general.
However, in blitz games, this rule is specifically excluded, therefore trying to flag your opponent is a valid winning strategy.
Personally, the reason I play with increment is for situations which are easy to win or draw "without thinking", meaning if the opponent does not play to win, I can just as well shuffle pieces around and therefore force a draw without losing on time,
Flagging - just like hitting people with balls in dodgeball - is a part of the game; if your opponent miss-manages their time, yet you don’t take advantage of it, then I would consider you to have lost on purpose.