There have so many undefined elements of the SEISS system, 'significant' being one of them, and each grant has differed from the last in terms of can you claim and should you claim. As you pointed out what people understand as significant will vary.
As with previous grants the word 'significant' only refers to the eligibility criteria, so whether you can or should claim at all - has the impact of coronavirus caused a reduction in your trading profits and do you consider that reduction to be significant for your business? So have you had less customers than you'd expected, cancelled contracts, have you been unable to take on work due to supplier issues? HMRC has never said to compare figures or used percentages to define reduced trading profits.
The significance of any reduced trading profits will differ wildly from one business to the next, for some any reduction would mean not being able to pay the rent, keep the doors open at all, for others it's just a number on a page. Only you know how significant any reduction in trading profits due the impact of coronavirus is for you at any given time. There is no percentage or figure put on this by HMRC, just a request for proof in whatever form you have should they ask for it.
The 30% reduction in turnover (not trading profits) when comparing two years only applies to the payment mechanism of the 5th grant. It wasn't used for the previous grants. It doesn't refer to the eligibility to claim or whether anyone should claim. It is there to define how much can be claimed if someone is eligible and needs to claim. The 30% is not a marker of how significant the impact coronavirus has been on your business when considering eligibility.
With regards to mortgages lenders will want to know your income stream is safe. Traditionally they're not the biggest fans of the self-employed in the first place and the self-employed generally have to jump through a few more hoops than the employed anyway. It's not in any lender's interest to wipe out a whole sector of new or existing customers and nearly 3 million people claimed one or more SEISS grants.. Equally it's not in their interest to take on risky customers. So while a few may shut the doors for a while, others will just want some reassurance that things have picked up or are back to normal or will want more detail. It may be a question of shopping around, using a broker etc if there are any problems, Those left behind, who were never able to claim anything and received nothing, those who have lost their businesses and those made redundant despite furlough will be facing a bigger problem overall.