Is it kufr to believe the angel Jibril made a mistake bringing revelation to Muhammed (saw) instead of Ali (ra)?

Yes definitely. The reason for this is the Angel Jibreel brings revelation directly from Allah. Suggesting he made a mistake is implying Almighty Allah selects an infallible creature to transmit his command rendering his revelation to become potentially disputed. Moreover, how can such a creation of Allah make such a mistake when Allah created the Angels free from sin, error or fault The Qur'an states:   يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ قُوٓا۟ أَنفُسَكُمْ وَأَهْلِيكُمْ نَارًا وَقُودُهَا ٱلنَّاسُ وَٱلْحِجَارَةُ عَلَيْهَا مَلَٰٓئِكَةٌ غِلَاظٌ شِدَادٌ لَّا يَعْصُونَ ٱللَّهَ مَآ أَمَرَهُمْ وَيَفْعَلُونَ مَا يُؤْمَرُونَ "They (the angels) do not disobey Allah in what He commands them, and do as they are commanded. (Qur'an 66:6)   Historically scholars cite a sect called Ghuarabiyyah that made such unusual claims which are regarded as kufr. For example, Ibn Qutaybah in Al-Ma'aarif (p. 623) states: غرَابِيَةٌ لِأَنَّهُمْ ذَكَرُوا أَنَّ عَلِيًّا كَانَ أشْبَهَ بِالنَّبِيِّ صَلَّى اللهُ عَلَيه وَسَلَّمَ مِنَ الْغُرَابِ بِالْغُرَابِ فَغَلَطً جِبْرِيلُ حِينَ بَعَثَ إِلَى عَلِيٍّ لِشِبْهِ النَّبِيِّ صَلَّى اللهُ عَلَيه وَسَلَّمَ بِهِ "Al-Ghuraabiyyah, they say that Ali resembled the Prophet (saw) like a crow with (another) crow, and Jibreel made a mistake at the time of the ba'ath, because Ali resembled the Prophet (saw)." and Ibn Qutaybah in Ta'weel Mukhtalif Al-Hadeeth (p. 157): ومِنْهُمْ الْغَرابِيَّةُ وَهُمْ الَّذِينً ذَكَرُوا أَنَّ عَلِيَّا رَضِيَ اللهُ عَنْه كَانَ أشْبَهَ بِالنَّبِيِّ صَلَّى اللهُ عَلَيه وَسَلَّمَ مِنَ الْغُرَابِ بِالْغُرَابِ فَغَلَطً جِبْرِيلُ عَلَيهِ السّلامَ حِينَ بَعَثٍَ إِلَى عَلِيٍّ لِشَبَهِهِ بِهِ "And from them al-Ghuraabiyyah are those who say that Ali (ra) from him was that he resembled the prophet (like) the crow with (another) crow and Jibreel (as) made a mistake at the time of ba'ath to Ali but he (Muhammad) resembled him. and Ibn Al-Atheer in Al-Lubaab fee Tahdheeb Al-Ansaab (vol. 2, p. 129): الغُرابِيَّةٌ وَهُمْ يَزْعُمُونَ أَنَّ جِبْرِيلَ عَلَيهِ السّلامَ غَلَطً فِي النُّزُولِ عَلَى مُحَمَّدٍ صَلَّى اللهُ عَلَيه وَسَلَّمَ وَإِنَّمَا كَانَ مَبْعُوثَا إِلَى عَلِيٍّ رَضِيَ اللهُ عَنْهُ "Al-Ghuraabiyyah claim that Jibreel (as) made a mistake giving revelation to Muhammad (saw) - he should have revealed it to Ali (ra)"