Is it tacky to only serve appetizers at an informal wedding reception?

No, tacky isn't really the right word...

'Relatively normal' perhaps, maybe 'nothing bad but nothing special' *shrugs*

If it's informal then informal food (still suitable for a wedding however) is to be expected, however appetizers are meant to be something small you have before a slightly larger meal - so maybe appetizers and one other slightly larger dish? I don't mean serve a full meal for everyone, but maybe have appetizers and something else...

Meh... the main thing is that it's not tacky, no.

Food is food I would do it

Tacky is only what one sees it as being (to me anyway).
It's the Bride and Grooms wedding. If they want finger foods or Churches Chicken it's all up to them. I'm going to have the basics - Ham Sandwiches, meat balls, chicken wing dings, baked beans & salads of some sort. I plan to keep it basic. I don't see the need to supply 5 star Entrées.

Of course not. I am assuming you will have drinks to go with the appetizers but a full course meal is not mandatory by any stretch of the imagination.
I have known people to serve only desserts at a wedding.
Whatever the couple wants to serve is their prerogative.
It's your day and the food served should be an expression of you.

We had a formal wedding and we did an appetizer buffet. People still tell us how much they loved the food to this day! I was that bride that didn't really eat much so I don't even remember what we served! I think if the food is good people don't care.

"only serve appetizers" as opposed to what