If we had a long enough, strong enough ramp, then maybe. However, the ramp would have to be so steep that it'd hardly be worth using when it's possible for a rocket to take off vertically.
So to answer your question, it could be possible, but it would not be worthwhile.
There are some ideas. Even a rocket sled could help, if you can refuel and reuse the sled easily. First stages burn fuel in the range of tons/second, if you can accelerate the rocket on a ramp for 10-20 seconds the rocket can get much smaller and cheaper or carry more payload at the same size.
The downside is the increased complexity, and you need some large mountain nearby.