Why decorate wall of mosque with calligraphy?

Answer:

In fact, it is an innovation - which is not good - mentioned by the Prophet Muhammad (peace be upon him) as one of the many signs of the last days.

People got it into their heads to decorate the masjid (mosque) with calligraphy. But it would have been better to remian doing what the Prophet did, which was to leave the walls pure so that they do not distract from worship.

Why we do not have paintings of people? because that is what idol worshippers do. It is forbidden by God. Christians do it because they are a faith mixed with Roman paganism. But Jews and Muslims do not. Ever.