Are you using pixelart?
When you create a new project there's a checkbox that says "optimize for pixelart" which sets a few settings.
You can also adjust these settings later in the project propierties
construct.net/en/make-games/manuals/construct-3/project-primitives/projects
- Fullscreen mode: Letterbox integer scale
- Fullscreen quality: Low
- Sampling: Nearest
- Pixel rounding: Set checked