Absolutely, there is a drop in quality as the zoom-ratio increases. This is normal because the designers have to made compromises when placing lens elements. This gives a performance which is uneven too. Many times you can have decent sharpness at one end of the zoom and it will truly terrible at the other end or somewhere in the middle.
The other thing is that ultra-zoom lenses are designed to be relatively small, even compred to a long lens with a similar reach. For example, there is no 18-300mm F/4. A 300mm F/4 exists and it is quite large, so the Sigma 18-300mm is F3.5-6.3 which means it is extremely dim at the telephoto end. That means much less light which reduces your ability to shoot as light diminishes. The 18-300mm weighs less than 600g while a 300mm F/2.8 weighs 2.4kg even though it does not zoom at all! It lets a lot of light in though and is optimized to produce top-notch quality at 300mm.