Mmm, it wasn't his acting or appearance that was bad, it was just that someone decided Scotty had to be the comic relief throughout, which ended up feeling a little patronising and didn't match up with the character we were all familiar with. Sure, in TOS, Scotty occasionally got drunk or cracked a one liner, but he could still be sober and professional. He was never a hairbrained, immature manchild. Even in the movies he only did two or three stupid things over the series.