yes and no.......it isn't the fact that she's a single mother that makes it settling.
If Rihanna had a kid right now, most men would still cuff given her looks, money, success, etc.
I think it's the image of the stereotypical single mother that makes it seem like a man is settling. Entitled, average looking, struggling to make ends meet, etc. If you're a man who has a lot going for him, getting with a woman like that might be settling. ...but a fine woman, who has a nice paying job, who doesn't feel like you owe her shyt just because she happened to get pregnant, and can already support herself without needing your money? that's not really settling, in fact in this extreme example she's still better than most non single mother options.
but if you're the type that thinks raising anyone else's kid is an L period, then it would be settling.