There is no single best month. The right time depends on whether you care most about visibility, long hiking days, cooler temperatures, autumn forest or quiet.
Spring
Longer days and fresh vegetation make spring attractive, but unsettled weather and wet ground can still matter. Treat summit conditions separately from the coast.
Summer
Učka is often used as relief from coastal heat, but exposed routes can still be hot. Early starts are the simplest upgrade you can make to a summer hiking day.
Autumn
Forest colour, lower sun and clearer-feeling air can make autumn exceptionally atmospheric. Daylight shortens quickly, so route timing matters more.
Winter
The mountain can experience winter conditions even when the coast feels mild. Snow, ice, wind and limited daylight can transform straightforward summer routes into serious terrain.