Precision: Mouse vs. Touch
A mouse or trackpad offers fine, pixel-level control when dragging and positioning an image, since the cursor moves in small, precise increments. A finger on a touchscreen is a much blunter instrument by comparison, covering more area and making tiny adjustments harder to land exactly. For especially detail-sensitive crops, a desktop input method generally makes fine positioning easier.
Screen Size Changes What You Can See
A larger desktop display lets you see more of the image and the crop frame at once, at a larger size, making it easier to judge whether a composition looks balanced. On a small phone screen, you're often viewing a more zoomed-out or cropped preview of the same editing interface, which can make subtle positioning issues harder to spot until after exporting.
Mobile's Real Advantage: Immediacy
What mobile lacks in precision, it makes up for in speed and convenience. If you've just taken a photo on your phone, cropping it immediately, without transferring the file anywhere, is significantly faster than moving it to a computer first. For quick, low-stakes avatar updates, that convenience often outweighs the slight loss of precision.
Browser-Based Tools Work Well on Both
Because a browser-based circle crop tool runs the same underlying code regardless of device, the core experience, upload, zoom, reposition, and export, stays consistent whether you're on a phone or a desktop. The main difference is just how precisely you can drag and position the image, which comes down to the input method rather than the tool itself.
A Practical Recommendation
For a quick, casual avatar update, mobile is perfectly fine. For something more deliberate, like a professional headshot or a brand icon, doing the final crop on a desktop, with a larger view and finer pointer control, tends to produce a more carefully composed result.