I'm just so used to using the pro controller in MK. I had a go online handheld and I just couldn't get the feel of it - it was cheese land which isn't the easiest of tracks but I was all over the place. I am not using tilt.
I've never used it like that but I imagine that might be OK. I've never used one joy-con on it's own. Does if feel cramped or not too bad?
Mario Kart is almost digital in its controls so I find the joy cons not to be an issue at all, and the Pro pad doesn't add anything for me in that game as finesse is not needed in the steering.