It needs to be declared on mortgage applications because student loan deductions affect your monthly income, in the same way that other commitments (e.g. money spent on groceries, dependents like children, if you have divorced and need to pay your spouse each month etc.) also have to be declared because they affect what you can pay each month.
But it doesn't really have any impact. For most people, disposable income is not the limiting factor. Especially if you don't have kids. For most people the limiting factor is going to be that the size of the mortgage can only be up to 4.5x / 5x your gross annual income.
If you have a reasonable credit history it's quite easy to get a loan in that range, whether you've got a student loan or not.