I wouldn't say
no problem, but it's a question of intent.
If a corporation microtargets an ad at me to let me know they have a product they believe I will like, that's a little creepy, but it might even be helpful. Maybe I'm in the market for a new backpack; maybe a company I've had a positive experience with before has just released a new model I might like. If they target an ad at me about their new backpack, and I buy it, that's arguably helpful for both of us...and at a minimum it's not deceptive. They're not trying to mislead me into thinking they have a backpack I might want, they're trying to ensure that I know the true fact that they have a backpack I might want. (False advertising is a legit issue, but it's also already illegal, already pretty well regulated, and not what we're talking about here.)
Similarly, an individual using ads to try and get hired is again trying to convey a specific message to a specific person using an unusual medium. The message, in this case, being "I'm cool, you should hire me", which is the same message every job applicant sends when they email their CV in. The message is fine, and the medium is merely novel; no big deal.
What we're talking about in this case is outright deception. Labour staff (allegedly) microtargeted ads to present a false perception; this is more like Uber's Greyball program than it is standard adtech. And, notably, the message wouldn't have been fine if delivered in some other medium. If Labour HQ had prepared a falsified spreadsheet of ad buys, and emailed that to Corbyn, that would be just as bad.