Yes, spelling matters in standardized English proficiency exams such as IELTS, TOEFL, and Duolingo English Test.  Spelling plays a crucial role in your IELTS marks but not as much in the TOEFL and Duolingo tests.  The TOEFL and Duolingo examiners are keener on the content of your answers rather than the spelling, but it is important to note that poor spelling will surely affect your marks in these three exams.