I’m mostly against doing this.
First, I might be wrong about Trump. But even if I’m right, there were many other people who were aggressive and unpopular during their own time but who as far as I can tell ended up being a net positive to their movements. FDR and Reagan seem like good examples here. I don’t think anyone is very good at predicting who will be in this category and who will be in the more Trump-ish category.
Second, even if Trump has caused a 5 pp shift away from populist attitudes and this was predictable beforehand, I think you would have to weight this against all the other potential effects of him being in power. Before his election, it wasn’t really clear if he was restrained enough not to start a great power war. Now that he’s demonstrated a tendency to concentrate on tweeting and not personally make major foreign policy decisions I’m a little less concerned; maybe there are people who could have predicted this beforehand, but I couldn’t. Progressives should also be concerned about his two Supreme Court appointments, his climate policy, et cetera. Even if I’m 100% right that Trump caused populism to lose support in a way other people wouldn’t, and even if this change is lasting, it has to be balanced against both his expected and his real downsides.
Third, making decisions like this creates weird incentives; if some candidates know you’re supporting the worst person with an ideology, in order to discredit that ideology, they will try to get worse.
I think these are important enough points that I would be very reluctant to try to make something like this happen strategically.
For a counterpoint, Nick Land believes the winning conservative strategy is to try to never win the presidency, then blame the (liberal) president for everything - see http://www.xenosystems.net/popcorn-activism/ . I think this could work, but only because Nick doesn’t really care about policy in a normal way and is just trying to maximize conservative popularity and/or lulz.
I am sort of tempted to apply something like this to Bernie Sanders, but only because I think it would be good for the country to be shifted towards a welfare state in the short-term, practical way that a president can do, and also good for socialism to be discredited in a longer-term more philosophical way. This is sufficiently weird that I don’t think it generalizes to very much else.