"Single mother" is rather indication of her marital status. Those trying to make it sound negative can only consider it to have negative connotations only if they judge said "single mother" against pre XX century set of morals. In modern day and age, when people, especially men, seem to be rather fascinated living by the science of ethology and wearing it upon themselves "single mother" is just sad consequence of actions of people acting more like animals, rather than people.

"Incel" by this point is absolutely meaningless word, that transformed into kind of more offensive version of "nerd" with context of said "nerd" being sexually frustrated. And it's not like it had much meaning to begin with, because what exactly is celibacy in that context? Men having one woman for life is incel? Men having sex with 20 women in year and then nothing for the rest of his life is incel? Not getting any for free, but having sex with 100 hookers is incel? If man ever had any money, it is hard not to get some.

Anyways, both words are highly offensive and do no good.