Is one who declares what is "makrooh شرعًا" (religiously discouraged) as permissible or impermissible, sinful, such as deeming it forbidden to enter the restroom with something containing the ذكر الله (remembrance of Allah)?

The reprehensible (makruh) is that which has been prohibited with a non-definitive prohibition. Sometimes, the term "reprehensibility" (karahah) is used, but what is intended is prohibition (tahreem) or that leaving it is preferable. If a ruling of reprehensibility is issued by a mujtahid scholar regarding a matter open to juristic discretion where disagreement is permissible, then there is no harm, such as the scholars' disagreement concerning the ruling on qaza' (shaving parts of the head and leaving other parts). However, if it is issued by an unlearned person or without knowledge, then it is impermissible and constitutes speaking about Allah without knowledge, which is among the greatest major sins. The four madhhabs have agreed upon the reprehensibility of taking anything that contains the mention of Allah into the lavatory, and they made an exception for the Mus'haf, stating that taking it in is prohibited.