How low ratio A260/230 can effect ligation and how can I increase it?

A260 value indicates nucleic acids (eg DNA) in your preparation. A230 shows proteins. If you have a low ratio that means your DNA prep is higly contaminated with proteins. This will reduce your ligation efficiency. You should use "proteinase" enzyme to digest proteins in the DNA prep.
Edit: If you are sure that you don't have any protein contamination (though some histone proteins on DNA will always be there even after gel extraction) then you have very low amount of DNA. You can also check this link suggesting butanol pelleting of DNA to increase the ratio.
http://protocol-online.org/biology-forums/posts/18324.html