I love pressure barrels (and corny kegs!) but a pressure barrel really isn’t suitable for lagers. Lager really wants around 20psi, needs to be very cold, and because of the high carbonation needs a smooth-flowing delivery system. The best you can expect from a pressure barrel is 10psi and the taps used on PBs do whip up the beer on the way out. If you’re not chilling the PB in a fridge to about 5C then the frothing will be even worse.
I think for this batch chill the PB if you can and maybe pour the beer in a jug so you can then fill and top up your glass with less foam from the jug. Next time maybe use bottles for lager (or invest in a corny keg setup) and use the PB for beers suited to lower pressures - bitters, pale ales, stouts, etc.
Good luck!