All of these numbers are basically made up anyway. TSS is a basic model of how the power that comes out of your legs causes physiological stress. Making up a number that matches your perception of how hard the effort feels is just a different model.
For sure the MTB effort felt harder than the TSS indicates. Similarly, I find that XC ski workouts feel harder than the HR-based training load indicates.
In both cases, for the purpose of tracking training load, I go ahead and use the TSS or HR calculation directly, without adjusting it upwards to account for how much harder the workout felt. But I also think it would be valid to adjust the numbers upwards. As long as the effort has some specificity, I think it's reasonable to say that how hard it feels is how hard it is. That's still a model, it's just a different model from using TSS religiously.
The reason I don't apply any adjustment is mostly that although I know the effort felt harder than the numbers show, I don't feel comfortable quantifying just how much harder, so it feels safer and more conservative to use the unadjusted numbers. In the back of my mind, though, I know that a lot of MTB or XC ski workouts probably leave me fitter than simple CTL would indicate.