The Spaniard has won twice, drawn once and lost twice since taking over from Marco Silva last month.
The Hornets are not out of relegation trouble just yet, but Mariappa is confident good results will continue under Gracia.
"I have really enjoyed working with him. He is very enthusiastic about football and always has a smile on his face and I think that rubs off on players," Mariappa said.
"I think you can see from the performances it has worked so far.
"He has had a positive impact on our performances and results as well. He and his staff are working very hard day in day out to get us results."