I don't typically have this issue at Walmart at least. Their self checkout is smooth and effective these days.
That said I still don't use it, because it's still shifting work to me without giving me compensation for it. If I got a discount for using the self checkout, sure, but I don't. So I'll keep using manned registers.
The Clark Kent disguise would definitely fool most people that don't regularly interact with either of them.
The real dumb thing is it's portrayed as fooling people who have a close personal relationship with him in both forms and spend non negligible amounts of time around both Clark and Superman. Those people would quickly notice.