I think the Surface 3 uses an Intel Atom CPU. Those are pretty weak so the weak link might actually be the CPU or the video GPU. The system might not have enough grunt for 1080/60. Also, I've not used Movie Maker but most video editing software has a preview in the editor and most of those previews are "draft mode" previews that don't look very good. So if it just looks worse when editing, I wouldn't worry about it. If it looks worse when the original MP4 or MOV is being played (by the TV and Movie Player or Windows Media Player), it might just be that the Surface tablets just aren't the best for video editing and it's struggling a bit.
Mike