As an old person I can tell you that that was normal humor in the old days.
It sure was. I was a teenager in the early '60s and making fun of the handicapped and people with, shall we say mental impairment (we called them "retards"), was an everyday occurrence at school.
That's exactly right. Maybe some of you were the picture of compassion and political correctness as adolescents, but growing up in the 50's and 60's it was common to joke around with your friends about being "retards" or making fun of some mannerism of theirs as if they were "retarded". I don't say this proudly, but rather to my shame and I have raised my children to be better than that. But still, it was how almost every one acted in those days.
I suspect Lennon was a product of his time just as we all are, except he also tended to be a jerk and egotistic to boot. But it is unreasonable to judge people's standards in an earlier age unless you factor in the times he lived in.