Hey Pete! Well actually, that's just a habit I developed playing acoustic gigs. I found that positioning the capo diagonally helped with tuning stability.

I was playing gigs where every song had a different fret the capo was on, and I noticed that if I put the capo on at an angle, the tuning wouldn't be so far off each time i changed it.

I've never even considered that it could help with the thumb. Does it actually help give you more room?

Thx!
Mike