This is a bit of a subjective question, but IMO yes, a Grails web application that is primarily a web service consumer is still a fine choice. Groovy's remote API functionality is very productive and clean, and the view layer and controllers are high quality regardless of what the domain/service layer looks like.
In my current project one of our Grails apps is acting just like how you're describing, and it's been a success in terms of quality, performance, and productivity.