There is no reason that a hypersonic missile can either have a conventional or low observable signature on radar, depending on the angle of observation, shape and OML materials used in construction. It would be difficult to make a really stealthy hyper sonic weapon considering the issues with atmospheric friction heating during flight at speeds in excess of Mach 3. I’m sure R&D departments within the superpowers of the world are expending quite a bit of time and national treasure on heat resistant low observable materials.
What are the big problems we face with hypersonic weapons today is the difficulty to mount an adequate defense to track and kill them once detected. Missiles in general have very low observable cross sections when viewed from a frontal perspective and, with hypersonic weapons traveling 5-20x faster than conventional projectiles, this doesn’t leave a whole lot of time for a killchain to work properly. In addition, some kinds of existing anti-missile weapons eg CIWS, etc. may not be able to effectively engage and kill a hypersonic projectile.
That’s not to say that anti-hyper sonic weapons, almost certainly and development, will not be able to do this in the future.