I don't think it is tacky to have people address their own cards and it is way easier.. People know you are going to be busy planning for baby so they shouldn't mind..
You're right, the purpose of a hostess is to make sure things are way easier for the guest of honor who just shows up to collect mountains of gifts.
It takes literally NO time for a mom to be to address and write her own thank yous and even less if the dad to be helps. If you want to make things easier for the mom to be, you volunteer to help her write, but for the love of all that is tactful, do not make the guests write their own envelopes. Honestly, I "forget" to fill mine out if I'm asked to do so because I'd rather not get a TY than get one that I had to write myself.