Yes, cover does affect melee attacks
The rules for cover are below:
Walls, trees, creatures, and other obstacles can provide cover during
combat, making a target more difficult to harm. A target can benefit
from cover only when an attack or other effect originates on the
opposite side of the cover.
There are three degrees of cover: half cover, three-quarters cover,
and total cover. If a target is behind multiple sources of cover, only
the most protective degree of cover applies; the degrees aren’t added
together. For example, if a target is behind a creature that gives
half cover and a tree trunk that gives three-quarters cover, the
target has three-quarters cover.
A target with half cover has a +2 bonus to AC and Dexterity saving
throws. A target has half cover if an obstacle blocks at least half of
its body. The obstacle might be a low wall, a large piece of
furniture, a narrow tree trunk, or a creature, whether that creature
is an enemy or a friend.
A target with three-quarters cover has a +5 bonus to AC and Dexterity
saving throws. A target has three-quarters cover if about
three-quarters of it is covered by an obstacle. The obstacle might be
a portcullis, an arrow slit, or a thick tree trunk.
A target with total cover can’t be targeted directly by an attack or a
spell, although some spells can reach such a target by including it in
an area of effect. A target has total cover if it is completely
concealed by an obstacle.
I'd consider the corner of the wall to be blocking at least half of that creature (it might be Bullywug) so it would get half cover from the attacker in the lower right.
I will also note that a melee attack victim could have cover if the attacker is attacking with a reach weapon and there is a creature between the attacker and the target.