Can one say l'shem mitzvas tiztzis in a bathroom? I was thinking of a case of tying the knots or fixing it. Since tiztzis is not a davar shebikedusha maybe it should be fine, or maybe since its mitzva related we want don't want you to think Torah thoughts in a bathroom.
The body of the garment that the tzitzit are attached to is not a davar shebekedusha, but the strings are very much so. In fact, the most mehadrin of strings are spun with tzitzit making in mind. Tying tzitzit is not a bathroom activity. Also, it is probably something that would be easier to do at a desk.