Seems like it is indeed much more offensive in the UK. I've never heard it used to mean "stupid" or "incompetent" in America. Here, it describes someone who might be scatterbrained, unorganized, eccentric, and a bit hyperactive or frenetic, but not necessarily unintelligent - you might describe your professor as "a bit of a spaz" without anyone taking offense, though you probably wouldn't say it to their face.
Seems like it is indeed much more offensive in the UK. I've never heard it used to mean "stupid" or "incompetent" in America. Here, it describes someone who might be scatterbrained, unorganized, eccentric, and a bit hyperactive or frenetic, but not necessarily unintelligent - you might describe your professor as "a bit of a spaz" without anyone taking offense, though you probably wouldn't say it to their face.