  We argue that the kilonova/macronova associated with the gravitational-wave
event GW190425 could have been bright enough to be detected if it was caused by
the merger of a low-mass black hole and a neutron star. Although tidal
disruption occurs for such a low-mass black hole as is generally expected, the
masses of the dynamical ejecta are limited to <~10^{-3}M_sun, which is
consistent with previous work in the literature. The remnant disk could be as
massive as 0.05--0.1M_sun, and the disk outflow of ~0.01--0.03M_sun is likely
to be driven by viscous or magnetohydrodynamic effects. The disk outflow may
not be neutron-rich enough to synthesize an abundance of lanthanide elements,
even in the absence of strong neutrino emitter, if the ejection is driven on
the viscous time scale of >~0.3s. If this is the case, the opacity of the disk
outflow is kept moderate, and a kilonova/macronova at the distance of GW190425
reaches a detectable brightness of 20--21mag at 1day after merger for most
viewing angles. If some disk activity ejects the mass within ~0.1s, instead,
lanthanide-rich outflows would be launched and the detection of emission
becomes challenging. Future possible detections of kilonovae/macronovae from
GW190425-like systems will disfavor the prompt collapse of binary neutron stars
and a non-disruptive low-mass black hole--neutron star binary associated with a
small neutron-star radius, whose mass ejection is negligible. The host-galaxy
distance will constrain the viewing angle and deliver further information about
the mass ejection.
