There are a lot of only fan-service manga, and it's ok. Many manga with no plot at all were created for the sake of fan servce.
But it annoys me in Fairy Tail, because the first chapters were more directed to all-audiences, a manga that keeps talking about the power of friendship as many shows directed to kids + teenagers. Then fan service would be about girls with big breasts and nice boys, not necessarily about showing everything, and now you should be at least over 13 to read it without feeling it's "too much fan service".
I don't read it any more, partly because of the fan service that is not necessary to this extent, and because I got tired of people who are about to die and are suddenly powered up by "the power of friendship" out of nowhere.