Great question! I actually think that the character of Tranio is one of the more interesting characters in the play for precisely the reason you have given. When we think of the common Shakespearian theme of appearance vs. reality, he is an excellent example of how this theme is developed. Note that it is he that comes up with the strategy to woo Bianca, rather than his love-struck lord. Also, if we look at Act I scene 2 carefully, which is when Tranio first enters playing the role of Lucentio, we see that he is able to speak convincingly as a Lord and is able to pun effectively to show that he has the graces, airs, and wit of a young Lord:
PETRUCHIO: Not her that chides, sir, at any hand, I pray.
TRANIO: I love no chiders, sir. Biondello, let's away.
Lucentio himself congratulates his servant in an aside for his convincing performance. He is able to fend away the requests of the other suitors well, making a speech with classical allusions to Leda and the swan. Gremio himself feels threatened by Tranio's gift of speech, as he says "this gentleman will outtalk us all." So, Tranio's act is seen as convincing not just by his master, but by other lords as well, especially when we think of his speech and his mannerisms.