Islam believes in humbleness and simplicity. Mosques during the time of the Prophet Mohammad (PBUH) were kept simple and the sahaba followed the same traditions and so did the Ta'aba and Taba Ta'aba.
Mosques are not to show your wealth and its not an art gallery. I know the christians even gold plate their cathederals and put very expensive statues and paintings in it but we muslims are different we will do what our Prophet showed us.
Plus instead of spending money on decorating walls......why not help the poor and give Zakat and khairat to the poor and needy! this is much more islamic than spending money on non living walls and windows..
I hope the confusion is clear now!