Time dilation is relative between you and some other reference frame. All the "stuff" in the universe nearby is (fairly) static, so we can say we are not moving fast, but it's still relative to the stuff around is.
It's similar to how the laws of physics are symmetrical under translation and rotation, but the presence of matter around us means the surface of the earth is not the same as the surface of the sun.
So, while we might use the CMB as a standard reference frame, the laws of physics should not.
If we didn't have any CMB radiation floating around to measure, everything would probably work exactly the same. Maybe you could measure all the motion of all the matter in the visible universe and measure the motion of the "center" relative to us.
There is no such thing. You always have to compare your speed to something else. Speed is calculated relative to something else. Relativity.
My point was that if we (the solar system) were travelling as a considerable portion of the speed of light, then we would experience time dilation, which would significantly affect our observations of the external universe.
We're not.
So... it's fine.
I mean, you're basically right; speed isn't absolute, and it's wrong to refer to your speed relative to the CMBR as 'absolute'.
...but, well... I think it's fair to say 'The speed of light is constant in all frames of reference, so why [insert question here]...' is a very fair, and common question. 'Everything is relative' isn't an answer, it's just a platitude.
This is significant for observing external events and correlating phase shifts to figure out distances.
Eg. Things in the external universe may appear to happen extremely quickly.
It’s actually very important.
Regardless of how fast you appear to be moving in your own frame of reference, the speed you’re travelling in absolute terms affects what you observe externally (and specifically with regard to how fast you are moving compared to what you observing)