It's not, but then the headline wouldn't subconsciously bias you towards rage clicking.

Infant only suspect in mother's mysterious death

They don't know she was a single mother. They don't know who she was because they weren't told. The father could have been on a business trip for all we know. That proposed headline injects one plausible guess as to why the baby would have been alone, but without any actual data to back it up.

It also fails to indicate that the baby was alone. You try to imply it by 'single mother', but whose to say that there wasn't someone else in the house that might have been important, like a sibling or extended family member? The detail that the infant was alone was key, but omitted in your version. Of course it's awkward to try to indicate the baby was alone without using the word 'left' in the context of a headline appropriate short sentence.

They could have injected the word "tragically" before dies for a bit more consideration, though people can see that the tragedy is pretty self evident.

I think that's a worse headline because it doesn't involve the sad imagery of a baby alone for 4 days. "Tragic" is a kitschy word to include in a headline. And neither your headline nor the one in the meme place the mother up-front.