Paul Hartley grabbed a consolation in the 82nd minute with a 35-yard free-kick in his 400th league start.
Di Matteo, whose side have now scored 11 goals in their last three games, said: "You can't be higher than top so after 17 games we can be pleased.
"But we will not be getting complacent.
As long as we stay at the right end of the table for as long as possible that will be good for us.
"It is still very close at the top and it can only take two bad games to put you right back in the mix. I will just keep going as this is still early days in my managerial career.
"The expectations are very high at the club and it is great to work for a club like this as I am very ambitious myself.
"I thought it was an excellent performance and I was delighted with the display.
We looked very strong.
"Getting an early goal is in your game plan and getting those goals at the start of the second half meant that the game was over."
City had arrived at The Hawthorns on the back of a seven-match unbeaten run and having lost just twice this season.
But there was no way back after Albion's quick double and City boss Gary Johnson said: "You can't give a team like West Brom a two goal start and then give them another two goals just after half time which killed us.
"We are very disappointed but we are not totally distraught after coming up against a very good side who are strong at home.
"We had a little bit of the play but didn't take our chances.
At 2-0 you know the next goal is vital and they got it.
"After a result like this you slaughter the players first because that is how you feel because we did defend poorly and didn't create enough opportunities.
"You let them know we are disappointed and they will take home a video and then come in and discuss whether they feel they have affected the game at this level as an individual.
"They have to be doing that at this level for us to be challenging for a top two spot.
"We have lost our seven-game unbeaten record and we will now have to rally the troops for the two home games that are coming up."