Can you use an iPad to open Apple saving account?

The new Apple Savings Account is reliant upon the Apple Wallet App - to which your associated Apple Card must be added. The Wallet App is only available for iPhone. As such, to benefit from the new savings account, you’ll need access to an iPhone:

Set up and use Savings in Apple Wallet - Apple Support

Whilst iPad does not have a Wallet App per-se, iPad does have Wallet in iPad settings. After adding your Savings account to your iPhone’s Wallet App, the account should be accessible on your iPad via Settings:

Settings > Wallet & ApplePay

To be absolutely clear - the Apple Savings Account will only be visible after it has been registered to your AppleID account via the Wallet App on your iPhone.

No, just from your iPhone:

Set up and use Savings in Apple Wallet - Apple Support