For Altera LPM_MULT to have registered output, is it enough to set latency to 1?
Setting the output latency to a non-zero value instructs the LPM to include output pipeline registers. Setting the latency to 1 will indeed correctly add an output register stage which will help to achieve better speed performance.

The "Resource Usage" box is really only advisory, and the actual usage will depend on implementation, device family, and optimisation settings. As such this value may not show an accurate representation.

In any case, if you design uses DSP blocks for optimisation, you will not see an increase in resource usage when adding up to 3 cycles of latency. This is because the DSP blocks internally contain register three selectable register stages - the first is at the output (latency > 0), the second on each input (latency > 1), and I believe the third is part of the accumulator internally (could be wrong).