You should look for the best combination of hard and soft skills in a mobile app designer who fits your budget. Their portfolio usually proves their experience working on Android, iOS, and cross-platform apps across different niches. You’ll want to corroborate the case studies with past clients via feedback posted on freelance platforms like Upwork, TopTal, etc.
Aside from their technical skills, you’ll also want a mobile app designer with good time-management and communication skills. The former is important for meeting your deadlines, while the latter is crucial for teamwork and avoiding misunderstandings throughout the design stage.