Losing to mugs is also underperforming, isn't it?Interesting though, Djokovic at the Australian Open and Nadal at the French were the only ones who stopped him. Your two stated exceptions. On grass he is utterly hopeless, so forget Wimbledon for him. Absolutely nothing should be expected of him there. Wawrinka was also fairly hopeless on grass, although still better than Thiem. He lost at the U.S Open in an epic he could have easily won vs Nadal in 2018. And made a great run at the WTF last year, just falling short in the end. So actually by your stated logic the only big event he underperformed for a whole 2 years now was the U.S Open last year.
Also with Djokovic at the Australian Open and Nadal at the French being exceptions, that already still would prevent him winning slams. To win the French you need to beat Nadal, to win the Australian you almost always need to beat Djokovic. Plain and simple. He can't win Wimbledon obviously. U.S Open is debateable, but his chances at the first two slams are far more.