Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Add butter and onion once the pot is hot. Saute until onion is soft and translucent, about 5 minutes. Add minced garlic during the last 30 seconds and cook until fragrant. Add in sweet potato cubes. Pour in chicken broth. Cancel Saute function.
Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 6 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Pour in heavy cream and season soup with salt and pepper. Stir to combine. Serve garnished with fresh thyme.