If you are not a Catholic can you be a godmother?

Traditionally, no - because a godmother was supposed to help the child's parents bring the child up in the faith.

An article on a godparent's duties says that now the role of godparent has become much more that of a friend and mentor than a spiritual guide. However, the christening service may require you to tell a white lie in answers about your religious beliefs. The article also states:

• Anglican Church law stipulates that a godparent should ideally have been baptised and confirmed, but discretion is often used to allow a godparent to stand even if he or she has not been confirmed, as long as they have been baptised.
  
• In the Catholic Church, godparents must be Roman Catholics who have received communion and been confirmed.

Maralee McKee gives a more secular answer:

"Being asked to be a godparent is a great honor. Congratulations! The obligations and expectations of godparents vary from culture to culture and from family to family. You could play anything from an honorary role to an integral part of the child’s spiritual upbringing. In general, a godparent’s role is to stay connected with the child in some manner throughout his or her life."

I think very many godparents take on the role with no firm intention of taking any further part in the child's religious upbringing and indeed the child can grow up not knowing who his or her godparents were, so in practice being a godparent is very often just a formality that allows the christening to proceed. In your case, you know how impractical such involvement would be, but nevertheless it is wonderful that you may want to take on an honorary role for a close friend. Only you (and perhaps the child's parents) can decide whether to be a godparent, knowing that you will not really help the child's parents bring the child up in the faith.