Is Bitcoin deflationary and inflationary at the same time?
The keyword is "excessive".

Inflation can be related to the supply of money with the quantity theory of money:  

MV = PQ  

Where M is the supply of money, V is the velocity of money, P is prices, and Q is production, all taken as the temporal quotient thus geometrically added. 

Prices can only go up or down.  Judging by the exchange rate, prices in terms of BTC are deflating as no one would pay 3 BTC for a gallon of gasoline as they would at the beginning of 2012 or so; now, they would pay 0.0003 BTC.

Price are deflating because demand for bitcoin is rising as evidenced by the skyrocketing velocity.

So even though the supply is growing, it is not excessive because velocity is growing so much faster.  Actually, the supply of bitcoin should've grown faster during this period, but it is not programmed to do that.

No cryptocurrency is currently programmed for perfect price stability.