/dev/mtd0 is the bootloader, so messing with it is generally what you do not want to do, ever.
If you manage to write s**t there your device will not be able to boot, not now and never again.
And they will not be able to fix that in Nokia Care, or in any service shop since it requires equipment present only at the factory and that was closed down a long time ago.

But the question is whether you were able to damage it?

If you are in Open Mode, it depends on how you did the writing. There are some hazardous procedures, but bootloader is locked down against simple intrusion.

If you are in Closed Mode and managed to tamper with it... well, then the odds are pretty badly against you.