Some people will want to talk about whether or not "My God" is an exclamation of surprise.
The trick here is not to discuss the meaning of the words used, but to point out that in context, Thomas is talking to Jesus who came back from the dead.
This means that all the so-called "blasphemous" statements that Jesus made are vindicated. Anyone can make blasphemous statements but not everyone can present themselves as risen from the dead afterwards.
For example here is one such statement from Mark 2:
Seeing their faith, Jesus told the paralytic, “Son, your sins are forgiven.”
6 But some of the scribes were sitting there, thinking to themselves: 7 “Why does He speak like this? He’s blaspheming! Who can forgive sins but God alone?”
8 Right away Jesus understood in His spirit that they were thinking like this within themselves and said to them, “Why are you thinking these things in your hearts? 9 Which is easier: to say to the paralytic, ‘Your sins are forgiven,’ or to say, ‘Get up, pick up your mat, and walk’? 10 But so you may know that the Son of Man has authority on earth to forgive sins,” He told the paralytic, 11 “I tell you: get up, pick up your mat, and go home.”
