Outside of online gaming, this is usually called sandbagging. In money games (e.g. pool, poker) it's hustling - you throw a game or two to make yourself look bad, then raise the stakes.
I haven't played World of Tanks in a couple of years, but in that game there were two issues:
- Smurfing, as described, where an experienced player creates a fresh account in order to look like a new player. This was used, in part, because of a 3rd-party mod that enabled people to scrutinize the stats of every player, in order to give them an overall quality score. With this mod, you could instantly identify the high-caliber players on the other team in a given battle. Those players could then be singled out for quick elimination. (Conversely, very poor players could be identified too, and you could ignore or quickly eliminate them en route to whatever your objective was.) So smurfing was sometimes a counter-strategy, to conceal one's skill from people who shouldn't (yet) know how good you are.
- "Seal-clubbing" was a separate issue: In World of Tanks, it's the vehicles that have grades, not the players. The players are all lumped together in 15-v-15 battles in a very Darwinian competition. When you first start playing, you're only able to play "tier 1" tanks, and you unlock tiers as you improve, using earned Experience Points as one of the game's currencies. Veteran players, with thousands of games played, are still able to play in those low tiers, where brand-new players abound (because brand-new players, by definition, can only play in low tiers).
Personally, I was much more offended by seal-clubbing than by smurfing in World of Tanks. I avoided playing tiers 1-3 unless I was working my way up a new line of vehicles (and being an experienced player with a pile of the game's in-game currencies, I knew how to zip through those low tiers). Tier 4 was where I felt, "okay, noobs, the game starts for real now - your [butt] is grass, and I'm a lawnmower." At those low tiers, I just felt it was rude to prey on people who literally don't know what they're doing yet, and it's bad for the game's growth. Who knows how many new players give the game a try and then say "f this"?