The best alternatives to PowerMockup are Peetch, Zen Wireframe, and SnapUp. If these 3 options don't work for you, we've listed a few more alternatives below.
Peetch is a 100% free PowerPoint Pitchdeck Template available for Mac & PC. - 42 layouts including device mockups - 172 icons (customizable) - Ultra light (788KB)
TeamSlide is a slide search solution for PowerPoint. It enables your team to find your best slides instantly, and always build up-to-date, on-brand presentations. We allow our customers to share knowledge, ensure consistent messaging, and drive productivity when building presentations.