In most cases, they are. But it can depend on specific interpretations and the views of the creators.
It's a bit complicated. Sometimes, elements from the light novels are integrated seamlessly into the main storyline and are thus considered canon. However, there might be instances where certain aspects are more ambiguous or open to debate among fans and creators alike.
Generally speaking, the BSD light novels are regarded as canon. They often expand on the world and characters established in the main series, adding depth and additional context that fits within the overall narrative framework.