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The DC to dispel true polymorph with Dispel Magic using a 3rd level slot is 19, I believe.
Dispel Magic using a 9th level slot automatically succeeds against True Polymorph.
Does the DC for a successful Dispel Magic decrease from 19 by using a higher level slot that is not 9th?
Dispel magic says:
For each spell of 4th level or higher on the target, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a successful check, the spell ends.
The only effect1 of upcasting dispel magic is that dispel magic can automatically end spells of a lower or equal level to the slot used:
When you cast this spell using a spell slot of 4th level or higher, you automatically end the effects of a spell on the target if the spell's level is equal to or less than the level of the slot you used.
Upcasting dispel magic has no effect on the spell DC. Only on the level of spell it can automatically dispel.
The DC only ever depends on the level of the spell you are trying to dispel.
1 - The only effect relevant to this discussion anyways. Of course it has side effects like changing how difficult it is to counterspell for example. (thanks @Slagmoth)
From the description of Dispel Magic:
For each spell of 4th level or higher on the target, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a successful check, the spell ends.
At Higher Levels. When you cast this spell using a spell slot of 4th or higher, you automatically end the effects of a spell on the target if the spell's level is equal to or less than the level of the spell slot you used
—PHB, pg234
Generally speaking, spells only do expressly what they say they do. Since the spell does not offer a means for scaling the DC based on the difference in spell slot levels, we cannot infer that there exists any such mechanism, RAW at least. The only thing altered by the spell slot level of Dispel Magic is whether the magical effects in question are automatically ended by Dispel Magic, or not.
This is my immediate thought from reading the spell description, but it doesn't feel right:
The DC to dispel any 9th level spell with Dispel Magic using any spell slot other than 9th is 19.
The only way around contesting the DC of any spell of 4th level or greater against a casting of Dispel Magic is to cast using an equal or higher spell slot resulting in automatic success. In this case, that would be a level 9 slot.
The spell description offers no option for a scaling DC. (But it feels like an 8th level slot ought to have a better chance than a 3rd level slot. May be an interesting house rule.)
