Well
Most lefties i know, my self included have awfully bad penmanship 
I'm a leftie, and my writing caused problems at school. In the mid to late 50s, my school expected everyone to write using a flexible nibbed pen dipped in an inkwell, we were made to write in a thankfully now dead style called copperplate, where hand pressure has to vary to splay the nib and widen downstrokes, it's probably the way that Charles Dickens used to write before he got an early Amstrad. I just couldn't do it, my handwriting was terrible. In those unenlightened days, the school tried to get me to write right-handed as well, but that was worse. My parents had long before accepted I was left-handed, dad bought me a book on italic handwriting, made me practice, and I never looked back, style, neatness and legibility at last. My school didn't like it though, and kept on trying to 'convert' me. Eventually, due to parental pressure, the school backed down and allowed me to write the way dad taught me, my handwriting is still pretty good today. As someone said though, some lefties are painful to watch, pen clamped in a bunched fist etc. I still use a fountain pen most of the time and hold a pen much as any rightie would, can't see why some find this so difficult. Fellow leftie Barack Obama was painful to watch, no idea what his writing is like, but it looks very awkward. You can criticise Trump for many things, but at least his handwriting is better.