Yes, they are. All memory foam mattress toppers are inflammable. That said, many legal jurisdictions have laws that mandate the treatment of memory foam toppers (and in some cases, all bedding products) with flame retardant chemicals.
However, the flame retardant treatment does not make the memory foam topper fireproof. It merely prevents the foam topper from lighting up like a match. That means you still cannot have a leisurely smoke in bed. Neither should you place lighted candles in your bedroom for a night of romance with your significant other.
Do note that not all memory foam bed toppers have this anti-flame treatment applied. Some may have been meant for export to locations with no flame-retardant laws for bedding, and some may be old stock from before the laws took effect where you stay. So you may want to check the label or other attached documentation as to whether the foam topper you want to buy has received this treatment.
Of course, maybe you do NOT want this treatment. Some people have reported that they have adverse reactions to the anti-flame chemicals commonly used. If you are willing to accept the increased fire risk, you may want to try looking for companies which make and sell “raw” memory foam. These companies make large slabs of memory foam in different thicknesses and sell them cut to your desired size. It used to be possible to find these companies using Google, but the new “improved” results seems to have eliminated them. You will likely have to resort to the phone directory like in the pre-Internet days.