The beautiful decorated wall made my day would be fine, as a sentence, if you added a comma after beautiful. However, in this situation it is unnecessary to change the adverb into an adjective when synthesizing this sentence. If you choose to do so, I would recommend keeping the word decorated, in order to maintain all the major elements of the original sentences.
A better option would be to maintain all elements of the original phrases exactly as they are. This means that instead of changing “beautifully” to “beautiful,” you should maintain the adverbial form. So, instead of eliminating the –ly and having The beautiful, decorated wall made my day, you can get rid of the confusion by keeping the –ly: The beautifully decorated wall made my day. By doing this, there is no possibility of deleting “decorated” in the first place, because the adverb must modify a verb or adjective. It cannot modify wall, which is a noun, and so you would need to keep the adjective decorated to have the sentence make sense.