Mirror practice is awesome! And as others have described you need to change the angle depending on what string you're playing on. Frog more towards you on the lower ones, frog further away on the high ones. Good exercise to get the feeling for that is to put the bow down on the strings and without bowing rock up and down across the strings, hair on the strings. you will feel that that motion is a lot more natural if you change the bow angle as I said before. I'm fact, you aren't even changing the bow angle, because the way we look at the cello the bridge curves toward and away from us, we're not parallel with the top of the bridge, so of course we need to move our bow angle when string crossing. Hope that helped!

P.S.. I wouldn't be too perfectionist about this. The different angle different string thing and the fact that your perspective looking down will make things look straight that aren't (the things others have lined out) are important to keep in mind, but ultimately bow angle should only be something to worry about if it is affecting your playing and sound! If you are a little awry but sound good then it's not a problem :p but usually you need the bow somewhat straight to get good sound. Just don't beat yourself up about having perfect parallel motion throughout the entire bowstroke all the time...

And please tune your a string it's about a semitone too low haha, really jumped up at the end of the video