I will answer the exact question you asked:
Is it okay to become a student with low grades?
Yes it is. You attend a school to learn - some learn better, some worse.
You attend a school in Germany where tuition is either free or (for non-EU citizens) cheap compared to countries such as the US. There is no miracle, money has to come from somewhere and it comes straight from my pocket in the form of taxes.
But this is fine: I strongly believe that any amount of education one receives is good and helps to wider one's mind. This is why I happily pay for otherothers to study.
The grades have zero impact on your future life - do not forget that school is just a step towards something you will be doing for the next 40 years (this can be academia, but also industry, ...). So learn and have fun.