Mockups are great if you want to post on social media. When the hackathon organizers ask you to upload the project to devfolio or any other platform which also asks for the screenshots, upload some mockups as well, it helps you stand out a lot more.
- Bendito Mockups - probably paid
- Previewed app
- Creatoom - Large category of free mockups
- Graphicpear - This would be so lit if I could afford it
- Shots
- Sindo - screenshot beautify tool