Marc Jacobs isn't above giving Luigi Mangione a style makeover for his upcoming New York state murder trial ... though he doesn't seem to think Luigi needs his help.

The famous fashion designer appeared on an episode of Variety's "Lie Detector Test" series ... when an interviewer brought up the bevy of celebs who have flashed Marc's designs at their court appearances -- including Winona Ryder, Lil Kim and Courtney Love.

The interviewer then asked Marc if he'd consider styling Luigi for his upcoming murder trial ... and he said yes.

However, when asked how he might improve Luigi's style, he admitted the guy has done a pretty good job dressing himself for court without any help.

Marc then agrees that Luigi is a "trendsetter" ... because Nordstrom apparently sold out of the maroon sweater fans believe Luigi wore in court.

While Marc might be down to make sure Luigi's dressed to the nines at his state murder trial, he may not get the chance ... because Luigi's defense team is trying to get the case dismissed.

As we reported ... Luigi has asked a judge to dismiss the case -- claiming he can't be tried again now that he pled guilty to federal stalking charges and admitted that he killed UnitedHealthcare CEO Brian Thompson.

Prosecutors in the case have until early October to argue the trial should proceed ... at the very least delaying the start of his murder trial if it isn't dismissed.

If Luigi does have to stand trial in the state case, at least he'll look good doing it!