To be honest, whether it was Howland or Lyanna, they both have issues. Howland was a small guy who, as far as we know, had never ridden in a joust or even worn armour like that before. Lyanna, being essentially a more-indulged version of Arya and even moreso after we see her in Bran's vision in ADWD, had probably been charging around Winterfell's courtyard in tourney gear before, so her being the knight is actually marginally more plausible than Howland Reed. Yet he/she took down three experienced knights at the joust without taking a serious injury or being unhorsed, which stretches credulity a bit.

The text in ASoS shows that Lyanna had kicked the arses of the squires already, though, and also that the challenge happened late in the day after the three knights had already been victorious and retired to their pavailions, not expecting to be challenged again. So it's quite possible they were drunk as skunks and easy meat for whoever it was