It's a dialect. "Ain't" is dialect for a lot of negated verbs. It can mean "am not", "is not", "are not", "have not", "has not", "do not", "does not" and, in this case, "there is not". The use of "nobody" here is also part of the dialect. In standard English double negatives are incorrect, so you can't say "There isn't nobody" to mean "There is nobody" or "There isn't anybody" (technically it wouldn't be incorrect to use it to mean "There is somebody, but no one would phrase it like this). "Who" is also omitted here. In some cases this can be done, but not here. I'm guessing this is also part of the dialect. In standard English, the sentence is "There isn't anybody who loves me better" or "There is nobody who loves me better".

No one can love me better. There is no one that can love me better.

There isn't anybody who loves me more.