I hope the matter is investigated properly, but I suspect that it was only one or two isolated incidents at most. I'm not aware of any reputable study that's ever shown voter fraud to be at any kind of level beyond a mere handful. In the 2010 election there were 232 total reports, of which well over half led to no further action; in 2015 the total number of reports was nearer to 500 but again most reports don't lead anywhere or are otherwise bogus. And incidentally those numbers probably include allegations made against candidates rather than just voters (see
https://www.electoralcommission.org.uk/find-information-by-subject/electoral-fraud/data-and-analysis for more )
I suspect this is the same level. No doubt there were a few stupid idiots who attempted to defraud the system but in the long run they are only a few and they shouldn't be used as an excuse to devalue the election result.
With that in mind, there were a few particularly close races this time round. North East Fife went to the SNP with a majority of just two votes, and there were eleven seats with winning majorities of less than 100. In those circumstances the result is highly sensitive to only minor changes, but if you flipped all those seats to the second party then the Tories would be only one seat better off. Unless you are determined to assert that the fraud was in only one direction, and so precisely targeted that you'd have to know the result before you rigged it, then I doubt that it would have been enough to affect the result materially. And a lot of the close seats this time round weren't even marginals to start off with.
No, I think the message is clear. Allegations of electoral fraud have to be taken seriously and I hope that anyone who did try such dirty tricks is brought to justice, but could it have made a big difference to the outcome? No.