Are Glyphs of Warding harder to find in dim light / light obscuration?

It’s up to the DM.

The rules for dim light and lightly obscured do not mention investigation:

In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.

But, the rules for advantage and disadvantage say:

The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.

So the DM May certainly rule that the check should be made at disadvantage if they think the environment warrants that.

There’s nothing written in the rules that explicitly tells you the check would be at disadvantage in a lightly obscured environment. It’s totally up to the DM.

Notably, heavily obscured imposes the blinded condition:

A heavily obscured area--such as darkness, opaque fog, or dense foliage--blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.

Blinded says:

• A blinded creature can't see and automatically fails any ability check that requires sight.

Since a heavily obscured area (such as total darkness) would definitely make it impossible to see the glyph, it seems quite reasonable that a lightly obscured area, such as dim light would make it more difficult to see the glyph and impose disadvantage on the investigation check.