You do need an aerial to receive digital freeview channels. The type and position will depend on the strength of signal where you live.
With digital and a good signal I have managed to run a freeview tv from a cheap set top aerial. Trying to do the same with analogue channels I cannot get a viewable picture.
The picture will never be poor on freeview. It will either be on or off. Although with a poor signal (aerial) you may see some image freezing or jerky pixelated images. In that case you will probably need a better aerial.