Comment by kgodey

sixdimensional covered this pretty well, but just to reiterate, I believe DBeaver is primarily a desktop app aimed at a technical audience. Mathesar is web-based and designed to have an intuitive, collaborative, spreadsheet-like UI for non-technical users who know anything about SQL or DB concepts. Also, Mathesar has deep integration with Postgres features e.g. you can configure roles and privileges through the UI.