Is snapped bolt on my motorcycles engine case bad?

I was putting the right engine cover back on my Honda Grom and overtightened one of the 10 bolts and it snapped with about half the bolt still in the hole. Is it fine to leave it and not deal with it or what do I do about it?

Mr Honda included that bolt in the design and manufacture process because they felt it was necessary. Hopefully it's one of the upper bolts and as long as the cover doesn't leak oil, you'll be right until you need to remove the cover again, in which case, remove the remains of the bolt. For future reference, the side cover bolts are only compressing the gasket material and do not need to be done up super tight. But you know that now I guess.

You remove it and replace the bolt. The case has 10 bolts because the engineers who designed it calculated that 9 isn't enough.

It will likely cause an oil leak. Fix it.

You can buy tools to remove snapped off bolts. In the UK they are called 'easy outs'. See link for example.

If it leaks oil take the cover off and try to remove the rest of the bolt with a vice grip pliers.
If the bolt snapped off flat with the case center punch the bolt and drill it out starting with a 1/32" drill bit and working your way up in drill size on size at a time until you can get an easy out in the hole.

Yes, it's bad.

It's NOT fine to leave it.
It's very UNfine to leave it like that.

What to do?
Take the right engine cover back off and get the broken bolt out.
I'd use a proper tap to clean the threads out.

If you don't have experience doing this, then pay somebody to do it properly. Don't even try to repair it yourself.

Trust me.

It is there to help seal off the gasket
if the bolt is above the housing, use multigrips to unscrew the broken part out

Unless you applied sealant to the gasket, it will leak. Every bolt is important. Remove the bolt and replace it. Look up the tightening torque and don't exceed that. Always use a new gasket.