Does bachelor's degree have to be related to a PhD subject?
I see that you question has two sides: 


The weaknesses of your bachelor topic (as you believe it is): In most of the cases, the bachelor quality is mainly evaluated for the eligibleness of joining a master program. After obtaining a master degree, there will be no concrete need to check the quality of the bachelor, rather the quality of the master thesis. 
Working in different topics/fields in bachelor, master and PhD: On the one hand, of course working on the same topic for several years makes you an expert in the field but with a very limited view. On the other hand, it is very common and normal to work on different topics. I personally know a lot of people who did their bachelors in a topic (e.g. Biology) and their PhDs in another topic (e.g. Computer Science).