Is it kufr (disbelief) if you do haram stuff in the masjid like play music or say bad words, or is it a major sin? Note that no disrespect was intented.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

There is no doubt that committing prohibited acts in the masjid is quite opposite to the very purpose for which the masjid was built; glorifying Allaah, The Exalted, by making thikr (remembrance of Allaah) and worshiping Him as befits Him. Moreover, such an act is contrary to the enjoined honoring of the sacred ordinances of Allaah; He says (what means): {...and whoever honors the sacred ordinances of Allaah - it is best for him in the sight of his Lord...} [Quran 22:32]

However, merely playing music inside the masjid with no intention of disrespect for the sacredness of the masjid is not kufr that takes the doer out of Islam. We could not find any scholars' statements indicating that a minor sin becomes a major one if committed in the masjid. They underlined, though, that sins deserve more punishment if committed in sacred places and during sacred times such as Ramadan.

Ibn Muflih wrote, "Chapter on the multiplication of reward and sin in the sacred times and places: Shaykh Taqiyy-Ad-Deen (ibn Taymiyyah) said, 'Sins committed during the sacred times and sacred places are multiplied just like rewards are multiplied therein in proportion to the virtue of that sacred time or place...'" [Al-Aadaab Ash-Sharʻiyyah, 3/431]

Please, refer to fatwa 81545 about the religious ruling on listening to music and fatwa 87671 about the importance of guarding one's tongue from foul language and inappropriate speech.

Allaah knows best.