There are a few reasons why this salad could end up mushy. You might have overcooked your veggies. This will make them mushy every time. It's important to keep an eye on them while roasting and remove them from the oven once they are tender but still have a slight bite. You might also have added the dressing too early. Adding the dressing in advance of cooking can lead to wilting and moisture accumulation, resulting in a mushy texture. Add the dressing to the salad just before serving for the crispiest veggies.