Use a video editing software to crop videos like Windows Movie Maker (eh tool though), Sony Vegas (industry standard like Photoshop) or even Blender's video editor. Also, Smash Ultimate's newest update comes with a video editor tool that can crop videos (which is why you should update your games for these sort of features than not for insubstantial reasons).
I have my Smash game on 3.0.0. I personally hate Ultimate's cropping tools. Cause you can't crop out the thing that comes after the results screen. Had no problem with that before the update. And for the Switch videos that can only have 30 seconds at a time, can use that feature to add other video clips on the PC. I'll also be cropping out swear words in the videos I make that involves me talking.