The layer creation option (LCO)
ENCODING=UTF-8 does not apply when creating GeoJSON layers. (In fact, it applies to few OGR-supported drivers, including ESRI Shapefile and MapInfo TAB.) So that option is being silently ignored.
According the GIS.SE question GeoJSON attributes encoding limitation, GeoJSON only supports UTF (including UTF-8, -16, and -32). In the reverse direction, the PostGIS database will only support whatever encoding was used when the database was created.
ogr2ogr will automatically convert between encodings where necessary, but in this case your database is in UTF-8 and GeoJSON only supports UTF-x. Therefore you should run this command without the
-lco switch, and you get what you get.
If you are not viewing the UTF letters as expected, this is an issue of your viewer, whether that be a text editor, desktop GIS, or programming language, not handling the UTF-8 characters correctly.