Do you feel that it is acceptable? Are their certain circumstances where you feel it is acceptable and others where you don't? If it helps, here are some reasons I have seen players do this:

• to end the game quicker
• the detrimental play is more "fun"
• for thematic reasons (and just want to stress again, I am only talking about competitive games)
• to show off (usually accompanied with a statement such as "I am still in the lead even after doing X")
• out of spite (specifically trying to make another player lose, while knowing it will also cause them to lose)
• sand-bagging/out of pity (Note that I would not consider establishing a disadvantage one player will be playing with before the game starts or trying out new strategies when playing with new players to be part of this catagory. I am talking about when a player intentionally makes a bad play out of pity without consent from the other players.)
• To make it less likely they come in "last place" (while also making it less likely that they win)

A conversation about the above situation came up in the Dominion sub-reddit and I was sort of surprised by the people's thoughts. I realize the specific context of that discussion may have affected people's replies, so I wanted to see what people thought about it more generally.