And had We appointed him an angel, We indeed would have made him a man, and We would have certainly caused them confusion in a matter which they have already covered with confusion.
meaning, if We send an angel along with the human Messenger, or if We send an angel as a Messenger to mankind, he would be in the shape of a man so that they would be able to speak to him and benefit from his teachings. In this case, the angel (in the shape of a human) will also cause confusion for them, just as the confusion they caused themselves over accepting humans as Messengers!
Allah said,
قُل لَوْ كَانَ فِى الاٌّرْضِ مَلَـيِكَةٌ يَمْشُونَ مُطْمَيِنِّينَ لَنَزَّلْنَا عَلَيْهِم مِّنَ السَّمَأءِ مَلَكًا رَّسُولاً
Say;"If there were on the earth, angels walking about in peace and security, We should certainly have sent down for them from the heaven an angel as a Messenger." (17;95)
It is a mercy from Allah to His creation that He sends every type of creation, Messengers from among their kind, so that they are able to call their people to Allah, and their people able to talk to them, ask them and benefit from them.
In another Ayah, Allah said;
لَقَدْ مَنَّ اللَّهُ عَلَى الْمُومِنِينَ إِذْ بَعَثَ فِيهِمْ رَسُولاً مِّنْ أَنفُسِهِمْ يَتْلُواْ عَلَيْهِمْ ءَايَـتِهِ وَيُزَكِّيهِمْ
Indeed Allah conferred a great favor on the believers when He sent among them a Messenger from among themselves, reciting unto them His verses (the Qur'an), and purifying them. (3;164)
Ad-Dahhak said that Ibn Abbas said about the Ayah (6;9),
"If an angel was sent to them, he would come in the shape of a man. This is because they will not be able to look at the angel due to light."
وَلَلَبَسْنَا عَلَيْهِم مَّا يَلْبِسُونَ
and We would have certainly caused them confusion in a matter which they have already covered with confusion.
meaning, We would confuse them over their confusion.
And Al-Walibi reported Ibn Abbas saying;
"We brought doubts around them."
Allah's statement,
وَلَقَدِ اسْتُهْزِىءَ بِرُسُلٍ مِّن قَبْلِكَ فَحَاقَ بِالَّذِينَ سَخِرُواْ مِنْهُم مَّا كَانُواْ بِهِ يَسْتَهْزِوُونَ