You'll pretty much always be better than where you started. It's just kind of how it goes. Of course, one can get better at consistency over time, but you'll still probably be improving and seeing things you can do better over time. ;D
Lot's of comics are long term projects, so for those that read long form, YES, it is normal and accepted for the artist to grow over the course of the comic. It's unlikely for anyone to be annoyed by this. If an artist is anxious by the quality change, it is always possible to create a new cover and stick that at the start with a little "I get better" disclaimer- it's not necessary, but I think it can assuage nerves, and it's generally better than redoing the whole beginning.
I will also say that this is why starting with a couple short projects before your main idea or a long form idea is a good idea. You can work out the more immediate kinks. But artists are always going to get better over time, so it's just not a big deal. Often times as a reader it happens so naturally you don't notice it if you're not looking for it. ^^