The 'Word of God' answer is no, they aren't.
Bob Gale, co-writer of all three 'Back to the Future' films stated in an interview that the various spinoff properties (including the BTTF games, comics, books and ride, etc) are not considered canon.
Voodoo Lordasks: Among the products that the trilogy spawned (animated
series, novels, ride, etc.), what are those you consider canon?
Bob Gale: Only the movies are canon. Everything else fits into the
‘what-if?’ category, or might be canon in some alternate universe.
That being said, Gale is pretty scornful of the idea of canon in general, as this interview makes clear...
"...the people at IDW said, "Can we say that these are canon,"
which is always something that makes me crazy. It's fiction. How can
it be canon? I said, "Look (and we actually have a text piece in the
first issue talking about this) when you have a time machine and
infinite time lines everything is canon, so, yes, it's all canon and
none of it's canon."