" does create the impression of what you want to mean, but generally dull refers to the poor capacity of a surface to reflect light (not shiny but dull), to hear less clearly (muffled or a dull sound), to a person's lack of intelligence (not bright, but dull), and also a blunt knife (not sharp but dull), you get the idea . . .
In movement or reactivity we would tend to say "sluggish
" which corresponds to a lack of vigour or lack of energy, slow to react. This is probably closer to what you are looking for.
By the way a string dies when it loses its elasticity, when it fails to give punch to the ball, fails to accelerate the ball. The string becomes sluggish, then it dies. A "dead
" string can be very bad for your arm, the ball stays much longer in the stringbed, the frame takes more and more of the shock, creating low frequency frame vibrations which can cause serious damage to the tendons, leading to elbow and shoulder problems.
One of the most frequent causes of these injuries is the abusive use of polyester strings by players who refuse to cut it out of the racquet after only a few hours of play.
Basic Polyester dies after a maximum of 4-5 hours of play, if you haven't broken it by then, cut it out and put in a new string, preferably in a thinner gauge. The thinner the better, it will be more lively, more comfortable and put more spin on the ball. The objective is to find the thinnest possible gauge that will give you 3-4 hours of play before it breaks, if it lasts too long, drop down a gauge, if it doesn't last long enough move to a thicker one. But it is much better to chose the gauge that gives you the best results, even if it only lasts 2 hours.
If you are not capable of cutting out the string when it is DEAD, then do youself a favour, put a mono core single wrap NYLON (a good SYNGUT in 16L is great) in the crosses, that will ensure that you maintain good elasticity in the crosses, even when the mains are dead. The hybride will stay reasobably playable from 2 to 5 times longer than a full poly restring.