It is fine to end a sentence with a comparative adjective?
It's better. It's faster. She's nicer. Or it's necessary to the sentence with a full comparison such as: It's better than it. This is faster than the second. She's nicer than her sister.
If you are comparing 2 things and the second thing is not understood, than X should be used.
Sometimes you are comparing the current state of something with its own previous state. Than is usually omitted since typically it’s obvious what the than X is.
I fixed the engine and now the car runs better.
Of course if you want to be extra clear, you can still use than X.
I fixed the engine and now it runs better than it did before.