If you have the 30% ruling you can deduct the mortgage interest under the standard regulations. So the 30% ruling itself will not affect the possibility to deduct the interest. It is also not relevant whether you have opted for partial non residency for tax purposes either.
The 30% ruling may however affect the actual refund. If your salary falls in a lower tax bracket due to the 30% ruling or due to the combination of the 30% ruling and the mortgage interest deduction, the mortgage interest will be deducted against a lower tax rate which leads to a lower refund for you.
Gross salary: € 70,000
Mortgage interest: € 10,000
Without 30% ruling
The salary of € 70,000 falls into the 52% tax bracket. This means that a deduction of € 10,000 leads to a tax refund of € 5,200.
With 30% ruling
The taxable salary is now € 49,000 (70% of € 70,000). This salary falls into the 42% tax bracket. This means that a deduction of € 10,000 leads to a tax refund of € 4,200.
These are just rough calculations but they do show that the 30% ruling can make a difference under certain circumstances. The same is also applicable for other tax deductions.