It doesn't mean that you've built too much capacity, it means that the producer estimates that his cost of shutting down some of the capacity is higher than just paying to offload the excess capacity and continuing to run at 100%.
That's an important distinction because you're always going to need more capacity, simply because peak use over the day tends to be 2x-3x more than during off-peak.
Whether negative prices are bad for producers depends entirely on the details of their setup. If they run a coal plant it's obviously mostly bad, since they're paying consumers to take their electricity while they're having to pay for coal.
But it's not necessarily going to be the case with other forms of production, e.g. what's the marginal cost of continuing to run a wind farm at 100% capacity v.s. shutting part of it down? Maybe shutting it down is expensive for some reason, and at that point you can just pay consumers to act as a heatsink, and make up the difference during the day.
Now, everything I've read on the topic suggests that excess negative prices are a bad thing today, but that's not some fundamental law.
E.g. let's say that fusion once invented is going to be ridiculously expensive to start up and stop, and can only run at either 0% or 100%, but it'll be cheap to just keep it running once it's started up.
Then you'd expect that there would be negative prices for around half the day, and for both the producers and consumers to be happy with that arrangement, since the alternative for the producer would be to buy a massive heat sink farm to burn off the excess capacity during the night.
Instead they could use the same money as a price signal to consumers to use their existing heatsinks, e.g. running their heating during the night.