Is it considered a sin to abandon someone who does not wear appropriate and clean clothes to the mosque and who makes many unnecessary demands, knowing that there is no sin in leaving him?

Your taking the man with you in your car to the mosque is an act of cooperation in righteousness and piety, and a recommended good deed. The Prophet, peace and blessings be upon him, encouraged giving rides to those who need them. In the Hadith: "Whoever has an extra mount, let him give it to one who has no mount." As long as there is space in your car for him, take him with you, and you will be rewarded for it. Do not deprive yourself of this good. However, if you dislike his company and do not take him, there is no sin upon you. You should gently and wisely advise him to wear his cleanest clothes and apply perfume when going to the mosque, in accordance with the Almighty's saying: "Take your adornment at every mosque."